They're treated like dice rolls. If you have "XdY+Z", "X" is the number of dice, "Y" is the number of sides on each die, and "Z" is the bonus on top of the roll.
So the range of damage is (X+Z) to (X*Y+Z). 1d2 would be 1-2 damage, and 2d2 is 2-4 damage. Numbers tend to gravitate towards the average between min and max.