Hooking up is used to refer to meeting someone at a party or bar and then later going to have some type of sex with them. Having sex with somebody you dont know very well, one time, after meeting at a social event. Making out or possibly having sex. Usually at a bar or at a party. Consequences.

What does hooking them up mean?

/hʊk/ us. informal. to arrange for someone to meet or work with another person or organization: We videotape applicants in practice interviews and hook them up with employers.