Guard, middle, middle & leg, two legs, leg, one leg

Before facing his first ball almost every batsman will wish to take his preferred guard - which simply means him lining up his bat relative to the stumps behind him.

He will hold the bat vertically, with the toe end touching the ground, and ask the umpire standing at the bowler's end for the guard he wants.
Common requests for guard are leg or one, for leg stump, two or middle-and-leg for a guard midway between leg and middle stump, and middle for middle stump itself. The umpire will tell him which way and how far to move his bat until it is in the required position, whereupon the batsman usually will make a mark in the right place on the popping crease with his bat or boot for future reference.

During a long innings a batsman may well ask the umpire to confirm his guard, or he may even change his guard in response to some change in the type of bowling.