In the most basic sense, turn on a person means to sexually arouse or stimulate them. This can be done through physical contact, visual stimulation, or mental stimulation.
Physical contact is usually the most direct way to turn someone on. This can include anything from kissing and touching to more intimate activities like sex. Visual stimulation can also be a big factor in turning someone on. This can be anything from watching an erotic movie or looking at sexy pictures to simply seeing an attractive person. For some people, mental stimulation is also a major turn on. This can involve thinking about sexual fantasies or situations, reading erotic stories, or even just having stimulating conversations with someone.