Your body produces a fluid (don't recall the actual name) due to the trauma experienced from the surgery. If the fluid is allowed to accumulate in your system, it could be fatal. So, they put a drain in that allows it to leave your body. It eventually stops producing itself and once it gets down to a certain level, the doctor can take the drain out.
In my situation, I knew about the drain from reading so much literature on it. I just failed to ask where. I had assumed it would be at the incision site, since that just made sense to me. No! I woke up and found out the drain was on top of my pubic bone (mons pubis) or whatever it is called. There's a tube going from a hole there to a little plastic thing that looks like a clear grenade with cc measurements on it. You have to keep track of how much you're draining out and tell the doctor. And of course, you keep pouring this thing out so it doesn't overflow itself.
Most people I've been told can get the drain out rather quickly. But, in my case, it took forever because my body kept producing enough of that fluid to keep it in. And, with the tube hanging down around my knees, it seems like everytime I moved my legs or knees I would pull the freakin thing and it would of course jerk that hole where the tube was coming from. That was miserable!! Plus, I have developed some type of nerve damage from that now, which is very painful. I've been told that some nerve must've gotten trapped from when that happened or something.
I would ask your doctor where they put the drain so you can be mentally prepared for that part of it. I have no idea if they put all of them in that same area or not.