The term refers to the checkers, and not the dice. You have a builder when you bring a checker into your outer board, where it bears onto a key point that you want to make. So, in other words, a builder is a checker which you use to build important points, planning your moves for the next rolls. As you can see, a builder helps you to build points, thus the basic meaning of the word does not differ very much from its meaning in backgammon.