The Property of Algebra which states simply

a(b + c) = ab + ac

An Example:

Pretend you made 8 nodes on saturday and you made 4 nodes on sunday. Each of these nodes gained you 3xp total, how much xp did you get in total?

3(8 + 4) = 3 * 8 + 3 * 4

          or rather

24 + 12 = The amount of xp you gained

This property is useful when you do not know one of the terms, for instance

6(a + 5) = 48

It would be time-consuming to continually plug numbers into this Equation to find the variable a without the distributive property, which in this case you probaly know by instinct.

6a + 30 = 48, therefore, a is equal to what integer?

a = 3

Note also that the Expression on the left is a Product while the Expression on the left is a sum. You should not forget the obvious fact that a Sum can be turned to a product just as a product can be turned into a sum.