Beats per minute (BPM) refers to the number of whole musical beats in a minute, and is used to measure the tempo of a song.
Inside the Timing tab of the beatmap editor, a BPM estimation tool can be found. This works by calculating a BPM and offset from the user tapping along with the song. This is useful for estimating a BPM value, but usually requires additional adjustment to be fully synced with the song.
Most modern songs use a single BPM which requires a single timing point. More complex songs may require multiple timing points and timing resets to be timed correctly. Timing points are shown as red lines in the timeline at the bottom of the editor.