Parentheses are used to specify the order of the computation.
As said, the
d operator is computed before of the
* operator, and the latter is computed before of the
+ operator. So, the formula "
2d6*3+4" means roll two six sided dice, add the result together, multiply it by 3 and then add 4.
Using parentheses it is possible to modify that order: whatever is put between them is computed before of the operators outside them. I.e. the formula "
2d(6*(3+4))" means add together 3 and 4, multiply the result by 6 and then roll two dice with a number of faces equal to the result. Or, in other words, roll two 42 sided dice and add the result together.