Original: ACE is, broadly speaking, a measure of the power of the hurricane multiplied by the length of time it existed, so storms that last a long time (such as Bertha) as well as particularly strong hurricanes (such as Ike), have high ACEs.
Simplified: ACE is a measure of the power of the hurricane multiplied by the length of time it existed, so storms that last a long time, as well as strong hurricanes, have high ACEs.