Intelligence refers to the possession of and, more interestingly for me, the ability to apply, transform or otherwise manipulate knowledge and skills.
As a trait, intelligence is very hard to pin down, and even harder to measure. Everyone has an idea of intelligence, but it's not measurable like the length of a toe. Instead, we need to make estimates based on our individual understandings of intelligence. Intelligence is an emergent property, so direct measurement would require measuring the properties of its constituents - but that wouldn't be very useful a lot of the time. So if we're not going to take a direct reading of a person's number of brain cells - which is impractical and also not very indicative of intelligence anyway - we must come up with other ways to try to find measurable things to fit into our immeasurable, human understanding of “intelligence”. This, in turn, gives rise to confused metrics like “IQ” and “EQ”, both of which try to make scientific approximations towards the kind of ability they want to make assertive statements on. IQ is determined by a test that attempts to isolate specific kinds of operations in the brain like pattern detection, short term memory, calculations, etc.. It really shouldn't be needed to be said that, beyond the obvious philosophical question of what even constitutes intelligence in the first place, these kind of tests performed in the most sterile environments imaginable should not be mistaken for actual intelligence, which even then very quickly hits the border of what people - not scientists of philosophers - regard to as intelligence. For some, just knowing a lot of facts is intelligence while, for others like myself, the ability to learn, retain and apply knowledge of all kinds and skills (not just fact knowledge, also kinesthetic skills, emotional skills, etc.) is much more important.
I am convinced that humans, and by extension their “intelligence”, are a product of their environments. Sure, genes form a base building plan from which to create a functional human being, but they don't prescribe “intelligence”. It would be kind of pointless, too, because if it was then every piece of gene would aim for the highest intelligence possible and there would be no differences at all. In fact, I believe that there are none to little. Sure, some genes may make you more predisposed for “exhibiting intelligence” later in life, but those do not “make you” intelligent or “cause” intelligence. Genes do not “cause” people to drown in pools, drowning in a pool is a circumstance that is emergent out of one's environment. While yes, there are genes that can make you more predisposed for falling into and drowning in pools, the drowning in the pool part has nothing to do with your genes and is merely the result of unfortunate circumstances. The evolutionary response to that would be to develop traits that make drowning in pools less likely, whatever that might look like.
Similarly, intelligence is not the result of genes. While some may be born with a setup of brain cells that will later help you exhibit signs of intelligence, intelligence is the result of development, studying and practicing. Nobody can look at a kid failing in school and go “yeah, that kid shouldn't try so hard, it's genetics, they are never going to be a rocket scientist”. Much better explanations are: Exhausted and overworked teachers, lacking fostering, stressful family/domestic situation, parental neglect (!), bullying, mental health issues, financial pressures (ie. poverty), discrimination, and other, many other external factors that lead to children failing to develop the necessary skills that will help them exhibit signs of “intelligence” later in life. Usually one of these is the culprit and, even in the absence of such, singling out genetics as the root cause will be impossible1).
“Exhibit signs of intelligence” - that's a weird way to put it. Yes.
The problem you will run into is that, in the real world, “intelligence” as a word is used very loosely to describe anything a person didn't expect to truly revolutionary insights that can change the world. So, what do all of these have in common? Well, a few years back I would have spent hours trying to find the “essence” of intelligence, but really it doesn't matter. Wherever intelligence is rooted, it is not a fact of the universe. It's a word used by humans to describe a pattern that exists only in their consciousness. Try to measure this using physical tools in the physical world. As far as practical application is concerned, we can only ever look at the behaviors exhibited by humans (or, provided expensive medical equipment, their accompanying brain), so all we can really say is “this person (or brain) is exhibiting signs or behaviors that match the pattern of intelligence as recognized in my consciousness”. Even if we take a holistic definition like “the ability to transform knowledge” then we can only see the end result, for example someone saying a thing that required a lot of knowledge transformation before such thing could have been said, but this only exhibits signs of intelligence, it never points at intelligence directly. In everyday life, the inflationary use of such becomes very apparent very quickly. People will call a lot of things or people “intelligent” or “smart”, I bet you have been called such a thing before, but really they couldn't know. They have seen you do one thing that may or may not be “considered” “intelligent”, but that will tell very little about yourself as a personality or your cognitive abilities. It's for this reason that I generally refuse being called smart or intelligent. It's happened several times before, but I do not believe that the person making that statement - or even myself, for that matter - really has the necessary knowledge to make such claims. I don't know, am I intelligent? Compared to everything I've seen, probably not, but what you are doing right there is in most cases usually just some weird expression of social conformity or something, I don't know. Like, I get that it is a compliment, but it's very shallow, almost insincere. Say less weird things. Try not simplifying reality into semantic concepts. (I have since gone from “strongly refusing to be called intelligent” to “generally disregarding where possible and when that is socially inappropriate, gently trying to pass up the compliment”.2))