A granule is an MP3-specific subdivision of a frame: each MP3 frame at standard sample rates contains two granules of 576 samples, each independently quantized and coded, sharing the frame's header and a pool of bits via the bit reservoir. The concept rarely matters until you try to edit MP3 sample-accurately — because frames carry encoder delay and padding and reference shared bits, you can't cleanly cut an MP3 on an arbitrary sample the way you can with PCM. Modern codecs use cleaner frame structures, so 'granule' is mostly a legacy term you meet when wrangling MP3 timing, gapless playback, or trimming.
