Rest
A rest is a sign for silence in written music. Each rest lasts as long as the note of the same name, and it is counted in the same way.
Music has silences too, and every silence has its own sign. A quarter rest is one beat of silence. A half rest lasts two beats and sits on the middle line of the staff. A whole rest lasts four beats — a full measure of 4/4 — and hangs down from the fourth line. The half rest sits, the whole rest hangs. An eighth rest lasts half a beat.
On a rest the key stays up, but the beat goes on: you keep counting, only silently. The notes and rests in a measure must add up to the beats in the time signature: a quarter note, a quarter rest and a half note make four.
A rest can stand anywhere in a measure — not only at the end, but on beat one or between notes.
What it sounds like
A rest is not a break: it is as much part of the music as a note. Well-counted silence keeps the rhythm just as firmly as sound does.
Where it is taught: the studio’s interactive textbook, chapter 2, “Counting: the beat, quarter and half notes” and chapter 13, “Ties, rests and the pickup”.