If point A is between B and C, then CA+AB = CB by the segment addition postulate. This is the idea of taking two smaller segments to "glue" them together to form a larger segment. Or you can think in reverse: take some larger segment and split it somewhere in between the endpoints (not necessarily the halfway point), this will produce two smaller segments.
For example: you have a ruler that is 12 inches. Take a saw and cut the ruler at the "2 inch" marker. You'll end up with two smaller pieces of plastic: one of which is 2 inches, the other 10 inches. The two smaller pieces can be taped together to reform the original 12 inch ruler.
