Beat
The beat is the steady pulse of music, like the ticking of a clock. We count it: one, two, three, four.
Music moves on a steady beat. Each tick is one beat, and the ticks are evenly spaced: never faster, never slower. The beat is the unit we measure notes with: a quarter note lasts one beat, a half note two, a whole note four.
Beats fall into groups. The first beat of a group is stronger, the others weaker; this pattern of strong and weak beats is the metre, and one group, from one strong beat to the next, is a measure. How fast the beats go is the tempo.
When you learn a new tune, count the beat aloud or switch on a metronome. On a rest the beat does not stop: the key stays up, but you keep counting.
What it sounds like
The beat sounds like even ticking or footsteps. It does not have to be heard to be there: a musician keeps counting it even when nothing marks it.
Where it is taught: the studio’s interactive textbook, chapter 2, “Counting: the beat, quarter and half notes” and chapter 3, “Measures and time signatures”.