() Are called parenthesis, and they are used to contain a part of a sentance or to seperate terms in math.
Examples:
I didn't want to explain (it was a long story).
2+3(2+1)
If the use is in math, you always do what is inside the parenthesis first.
For example:
2 * 2 + 2 + 1
4+2+1
6+1
7 is the answer when no parenthesis are added.
2 * (2 + 2) + 1
2 * 4 + 1
8 + 1
9 is the answer when parenthesis are added, because you do the operations inside of parenthesis first.
Hope this was helpful!