Redbook is the 1980-defined standard fro Audio CDs.  It defines CD audio as 44.1khz, 16-bit linear PCM-encoded audio, with a number of parallel tracks called P, Q, R, S, T, U, V and W which encode various bits of data useful to the CD.  P and Q codes in particular handle things like “track start” and “track end.”

There’s also “orange book” and “yellow book” and a few other colors used for defining other CD standards, like CD-ROM, CD+G, etc.