All Trait tests and damage rolls in Savage Worlds are open-ended. That means that when you roll the highest number possible on a die (a 6 on a d6, an 8 on a d8, and so on), you get to roll that die again and add it to the total. This is called an “Ace.” Any modifiers to the die roll should be tacked on after adding up an Aced roll.

Savage Worlds Deluxe Edition p. 70