The single paradiddle is a four-note sticking pattern: R L R R (or its mirror, L R L L). It combines a single stroke and a double stroke into one phrase. Once you can play it cleanly, you can move accents around inside it, alternate hands across the kit, and use it as the foundation for hundreds of fills.
Why it's so useful: the pattern naturally alternates which hand starts each grouping, so over four reps both hands lead twice. That balance is the whole point.
The Pattern
Eight 16th notes per bar, sticking R L R R · L R L L — that's two paradiddles, one starting with each hand. Loop it.
Exercises
1A — Single Paradiddle on Snare
Sticking: R L R R · L R L L · R L R R · L R L L. The accents (the > marks above the staff) naturally fall on counts 1, 2, 3, 4 — the start of each paradiddle group. Let those notes be slightly louder than the rest.