What does it mean to "track a beat"?

If you’re talking about audio recording and the music production process, then tracking a beat means the composing, arrangement, and recording of the initial “backing tracks” of a song; in the pop/R&B/hip-hop genres, this is the initial preparation and recording of the rhythm section (drums, kick & snare, bass line, backing rhythms, samples, etc.) for a song.

As a next step after the tracking of the beat, the ‘live’ performers come in and lay down their tracks, usually the lead vocals/lyricists, solo instruments, and embellishments.

Finally, the whole “multi-track” recording is then edited, blended or “mixed” down to a master (usually a single, “final” stereo recording), to be incorporated into an album (collection) of songs, which are then “mastered” collectively for consistency and technical quality, before calling it a “finished” LP, EP, etc.

That’s the long and the short of it.