There are 2 main definitions of hook up in English:
(1) informal (Of two people) meet or form a relationship: he hooked up with a friend in Budapest
Within days, you'll be able to find photos, download songs and hook up with friends you met at the show.
I've been in two long serious relationships, and hooking up with handsome slightly drunk rich kids was exactly what the doctor ordered.
(2) Engage in or form a casual sexual relationship: hooking up with total strangers can be very dangerous
I didn't feel guilty, or satisfied that I'd finally hooked up with my dream girl.
(1) A casual sexual encounter: he was tired of meaningless hook-ups
The article added that she has since been engaging in "meaningless hookups - at least four in the last month".
(2) A person with whom someone has a casual sexual encounter: she’s hurt that her hook-up didn’t even bother to stick around long enough to say good morning
He hasn't seen his son in months; he's divorced; his one-time hook-up looks like she's going to remain a one-time hook-up.