Hookup are the meetings between couples and other couples or singles for NSA (No Strings Attached) sex.
There is a huge shortage of females in India, so there are millions of men who are unlikely to ever have a life partner. Hookups are a way for them to have sex in a safe environment with couples of ladies who like to have multiple partners.
Men: Please be aware that Hookups are for sexual fun only. They are not meeting with potential life partners.