Expressions are constructions of natural numbers and dice rolls. The expression is always evaluated following PEMDAS order.

1d20 + 2d6 * 2

Parentheses work as you'd expect them too.

(2d6 + 6) * 2

Prefixing a die roll with ^ will roll two of the dice, and drop the lowest. Prefixing with a v will drop the highest die roll.

^d20 + 12