If a plosive sound has a fairly long positive VOT (longer than about 50ms), during this period air from the lungs is travelling quite quickly through the vocal tract. It is not slowed down either by the vocal folds (which are open) nor by a constriction in the vocal tract (because the plosive has been released. The rapid airflow creates a weak friction noise. This is aspiration.
Here are some sound clips of plosives with negative, zero and positive VOT. The positive ones are aspirated.