How old are you btw? *I'll admit, it's not the best way to start a reply! *
It depends on the context - if it's a friend of many years or group of friends asking if you want to go out for a drink, then normally it's a way of catching up on news & gossip as well as reminicing about the past.
If it's a guy asking a girl if she wants to go for a drink, then it's a way of getting to know someone before dating them and then getting into a relationship. Eventually.
HOWEVER, *and this is important* some guys use this way as getting girls drunk and then getting laid with no intention of seeing them again. Or just seeing them for sex.
So be careful. Go somewhere that you know, tell your friends about where you're going and don't forget, you can ALWAYS say NO - alcohol, unwanted advances, etc.
But saying that, when you get right down to it, it's a way of one person to get to know another in a social environment.
If you decide to take him up on his offer, enjoy!