Sexual (also allosexual) is the opposite of asexual. That is, a sexual person (noun form: a sexual) is a person who experiences sexual attraction. They can be heterosexual, homosexual, bisexual, pansexual, or various others.
"Sexual" has also been used to mean a variety of other things, including having sexual behavior, sexual desire, a biological sex, a gender, and a sexual orientation. This sometimes confuses people who use a different definition of "sexual" and then assume "asexual" means "not sexual". But in the asexual community, the words "sexual" and "asexual" are usually just used to distinguish between people who experience sexual attraction, and people who don't.