What does the law of conservation of momentum mean?

"Momentum is conserved in any collision if the effect of any external forces present is negliable relative to the effect of the collision."

To explain it a different way; imagine playing pool. If you have good form, and hold the Cue-stick properly, and follow through on a straight-shot, then the cue-ball goes straight, and it collides with, say, the eight-ball.

If a straight imaginary line can be drawn down the length of the stick, and through the very center of the cue ball and through the very center of the eight-ball... then the collision of stick and cue-ball, and the collision of cue-ball and eight ball will result in all of the from the cue-stick travelling first ball into the second, and the eight-ball will travel in that same straight line (assuming all other forces in the game, like the felt, the angle of the table-top, are all negligible.)

Another example is the famous "Newton's Cradle":

Imagining two objects colliding, the results are the same, if a little more complicated. You see, Momentum is the result of multiplying mass and velocity (which includes a direction, a vector).

So when two objects, each with mass and force, collide, the momentums of both objects can change, but the sum of the momentum's remain the same.