Preferential describes treatment, prices, or rules that are not the same for everyone: they are made different on purpose, so that some people get a better deal. Preferential treatment means you are put first, or given extra help, because of who you are — a friend, a regular customer, a country, a group the rules want to help. Preferential rates are cheaper prices for some people (students, members, certain trading partners). It is a rather formal, news-and-policy word. Though actually, it can sound kind (“they give preferential rates to older people”) or unfair (“he got the job through preferential treatment”). The point is not “I like this”; the point is “the system is tilted on purpose.” Preferred is a close cousin in phrases like preferred customer, but preferential is the adjective for that tilt. You will also meet it in trade talk (preferential tariffs) and, in a more technical stretch, in science, where one thing is taken up or chosen ahead of another. Everyday English often just says “they treat X better.”