So far you've cut beats into halves (8ths) and quarters (16ths). What if you cut a beat into three? That's a triplet. Three even pulses fitting into the same time as two of the next-bigger note: an 8th-note triplet is three 8ths in the space of two. They get marked with a small "3" above the beam.
Triplets are a different feel, not just a different count. Where straight 8ths split a beat into two equal halves — boom-boom, a regular pendulum — triplets split it into three, with a rolling, lopsided forward motion. The classic count is "1-trip-let, 2-trip-let, 3-trip-let, 4-trip-let," three syllables per beat.
Triplets also unlock something huge: the swing 8th. If you take an 8th-note triplet and only play the first and third partial — skipping the middle one — you get a long-short pair that sounds like a swung pair of 8ths. That's how jazz drummers, blues drummers, and shuffle drummers actually subdivide. On the page it might be written as a regular pair of 8th notes with a "swing" instruction at the top — but in the player's head, it's a triplet with the middle missing.