Personal preference are your tastes, likes, dislikes and benign opinions about things that don't need higher justification. For example, you don't need higher justification to say that you don't like red cars. Some of us just don't like the color red, especially on cars, and that is a perfectly valid opinion to have.

Personal preference inhabits an interesting niche in philosophy, as you cannot directly use it as a prescription to tell other people what to do (or, especially, what not to do), while at the same time it constitutes a Claim for yourself¹⁾. What I mean is that you cannot say “I don't like red cars, therefore red cars should be banned”. However, at the same time, someone else can go “I like red cars and I want to have one”.
The person who doesn't like red cars will now have a red car forced upon them (ie. they will have to tolerate a red car in traffic), which is a form of prescription²⁾.

The general idea seems to be that the people who like red cars gain more pleasure from driving them than harm is caused to those who dislike it. Therefore, the personal preference of the red-dislikers is overruled by the personal preference of the red-likers. To do something about red cars, more/better harm-based argumentation would be needed. “Higher justification”, ie. you could for example raise safety concerns (“red cars stand out, visually, which may distract or disorient other vehicles in traffic, especially during a high-stress situation”). This has not happened, therefore red cars get to drive, whether you like it or not³⁾.