A drag is two quick grace notes, a double stroke from one hand, just before a main note from the other hand. It sounds like a short, soft ruff leading into the note.
Sticking
llR rrL
Capital letters are main notes. Small letters are grace notes played just before the main note.
How to practice it
Play the main notes alone first, without the drags.
Add the drag as a soft, close double stroke from the other hand.
Speed up only when the drags stay quiet and the accents stay clear.
The two grace notes should be quiet and close together, almost a buzz, with the main note clearly louder.
Where you will use it
Drags add texture to snare backbeats and are the base of the drag family of rudiments.