The basic rhythmic pulse of a track; in standard 4/4 dance music each bar contains four beats, with the kick drum typically landing on beats one and three.
A beat is the smallest standard unit of rhythmic time in a piece of music, representing one quarter-note pulse in standard 4/4 time. In most dance music, four beats make up one bar, and the kick drum typically falls on beats one and three while the snare or clap falls on beats two and four.
Why it matters
Every higher-level structure in DJ mixing, including bars, phrases, drops, and transitions, is counted in multiples of beats. Losing track of where beat one falls in a bar means losing the ability to mix in or out on phrase boundaries, which is the most common cause of a structurally awkward transition.

