A flam is two strokes that land almost together: a quiet grace note just before a louder main note. The grace note comes from the other hand, which gives the flam its thick, wide sound.
Sticking
lR rL
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 grace notes.
Add the grace notes, keeping them low and quiet, about an inch off the head.
Speed up only when every flam sounds like one thick note, not two.
Start with one stick high and one stick low. Drop both together; the low stick lands first as the grace note.
Where you will use it
Flams fatten up backbeats and accents, and they are the basis of the whole flam family of rudiments.