Skill Check:
d20+ Skill Modifier, againt a certain DC decided by your GM, it is successul if the total result is equal or greater than the DC.
Also whenever you roll one or more d20s and add a Skill modifier to a roll, that roll is also considered a Skill Check.
For example, an Attack Roll is also a Skill Check, however a Damage Roll is NOT a Skill Check, since it does not roll a d20.