What is distributive law? I read about distributive law of addition and multiplication.
I could not under stood them very well because I am self teaching myself algebra.

It's just a basic property of given set. It states that for all numbers in that set (more specifically ring)  holds the same property. 
x · (y + z) = (x · y) + (x · z) or (y + z) · x = (y · x) + (z · x);

As you can see more common algebraic structures like $\mathbb{R}$ or $\mathbb{Z}$ (real or integer sets with predifined multiplication and addition) holds that for all their elements. 
Considering strict algebra, sometimes we have distributive law with not just multiplication and addition. It can be addition and multiplication modulus or something else. The most crucial thing is for that two operation to satisfy the equality above (with $+$ and $\cdot$ operations changed to the one you want/need). 
