Ability checks are used when a character doesn?t have any ranks in a particular skill and tries to use that skill untrained. Note that some skills can?t be used untrained.
For example, if your character wants to bluff a couple of Imperial officers but doesn?t have any ranks in the Bluff skill, the GM calls for a Charisma check (Charisma is the ability associated with the bluff skill).
Ability checks are also used to determine success when no skill applies. Trying to move a heavy cargo containers, for example, requires a Strength check.
To make an ability check roll a d20 and add your character?s modifier for appropriate ability. If the result is equal to or greater than the DC of the task, the check succeds.