Knowing your gaps is crucial to being able to run the football. You can't really run if you do not know where you are going. Take a look at the basic gaps and learn where they are.
You will quickly notice that the numbering system starts from the center and goes right and left of him. To the right of the center you have the even numbers, to the left you have the odd numbers. The center himself is the 0 or 1 gap, depending on which side. If you run right at his right butt cheek, you are at the 0 gap. Anything wider than that is the 2 gap. Likewise for the other side.
The running back is usually given the number 2. The fullback is usually number 4. So if the play is a 28 sweep, that usually means that the number 2 running back is running a sweep to the 8 gap, way outside on the right. Very easy to understand.
If you call a 41 trap. The play is a fullback trap to the 1 gap, right at the left butt cheek of the center, with a possibel fake or follow through to the running back.
If you call a 25 counter, the running back (no. 2 back) will be taking the ball as a handoff to the 5 gap on the left, but since, it's a counter, he takes a step to the right first to freeze the linebackers.
With a numbering system it makes life a lot easier for play calling. You should quickly be able to figure out who is getting the ball, where you are going, and to what side.