Rhythm
Rhythm is the pattern of long and short notes and rests that plays over a steady beat. The beat is even; the rhythm on top of it can be anything.
Beat and rhythm are not the same thing. The beat ticks evenly, like a clock. Rhythm is how the notes fill those beats: a quarter note lasts one beat, a half note two, two eighth notes split one beat in half, and a rest is counted silence.
We count rhythm with numbers — one, two, three, four — and eighth notes with an “and” between the beats: one, and, two, and. Many music schools also read rhythm in syllables: a quarter note is ta, a pair of eighths is ti-ti, a dotted quarter and an eighth is ta-a-ti.
Read a new rhythm before you play it: count it or say the syllables first, then go to the keys.
What it sounds like
Rhythm is what you can clap without any melody at all. Many tunes can be recognised from their rhythm alone, even on a single note.
Where it is taught: the studio’s interactive textbook, chapter 2, “Counting: the beat, quarter and half notes” and chapter 12, “Eighth notes and the dotted quarter”.