First of all, welcome to the forum, <img src="/ubbthreads/images/graemlins/wave.gif" alt="" />
5 + 10 extra damage would mean a spell or weapon inflicts 5-15 damage upon an enemy.
24 + 20 extra damage = 24-44 damage.
The first number represents the minimum number of damage a spell or weapon inflicts on an enemy when there is a successful hit. The two numbers, when added together, tell you the maximum amount of damage a spell or weapon is capable of.
Thanks.