A flam is a rudiment involving two single strokes: a grace note followed immediately by an accented primary note on the opposite hand. The grace note is really just an ornament that adds to the strength of the primary note; both happen almost simultaneously.
The two stroke types involved in a single flam are:
A tap OR upstroke for the grace note, depending on what that hand needs to do next.
A down stroke for the primary note.
If you hold the first hand in the down position, and the second hand in the up position, then try to strike the drum at the same time with both, a flam will probably be produced! The first stick has less distance to travel, so it will sound softer and strike just before the other; the second stick will sound stronger and slightly later.
Alternating flams between the hands should feel like a seesaw motion. The grace notes need to be upstrokes to prepare that hand for the next primary note.
A ruff is like a flam, but with multiple grace notes played by the first hand instead of just one. A drag is a double-stroke ruff. Other ruffs can incorporate more than 2 grace notes using alternating singles.
To take this challenge to an even more advanced level, change all of the flams to drags!!!