Refer to figure.
The balance ball (or slip indicator), is actually a device called an "inclinometer". It is a metal ball inside a curved tube filled with a damping liquid (usually gasoline, as its viscosity is correct for this application), and there is a small tube linking the left and right side (out of sight of the pilot) for easy fluid flow.

The ball is inclined by gravity to sit in the centre of the tube, and this is the case when stationary or in coordinated flight (no sideslip). The ball moves to either side of the centre due to extra forces acting on the aircraft, and these forces usually indicate that the aircraft is in a sideslip condition. For instance, if the aircraft is sideslipping to the right (air coming from the right of the aircraft) in straight and level flight, then the balance ball detects the force on it (in this case right wing down), and sits to the right of centre. To get back to coordinated flight, the pilot must "step on the ball", in this case applying right rudder. After the correct amount of rudder is applied (and held), the aircraft will resume coordinated flight.

This is most useful during a turn, when it is easy to apply not enough, or too much, rudder to coordinate the turn. In a left turn, for instance, balance ball to the left means you are in a slipping turn and need more left rudder, whereas a balance ball to the right means that you are in a skidding turn and need to apply a little less left rudder.

Therefore, with a balance ball to the left of centre, apply left rudder to regain coordinated flight.