Everything else on this site is about which notes to play. This lesson is about when. Rhythm is the half of music that scales cannot see, and it is the half your body already understands.
The beat is the steady pulse you nod to. Its speed is the tempo, measured in BPM, beats per minute. 60 BPM is one beat per second, a slow ballad; 120 BPM is a brisk pop song; 180 BPM is punk sweating.
Beats are grouped into small boxes called bars. The time signature tells you how many fit in each box. 4/4 means four beats per bar, and it earned its nickname "common time" honestly: the overwhelming majority of pop, rock, blues and electronic music lives in it. 3/4 is the waltz, one-two-three, one-two-three. 6/8 rolls like a lullaby.
Count 1-2-3-4 along with any rock or pop song and listen to the snare drum: it almost always cracks on 2 and 4. That is the backbeat, and it is the single most recognizable rhythmic convention of the last seventy years. Clap on 2 and 4 and you sound like a musician; clap on 1 and 3 and you sound like a wedding guest.
Open the progressions page, build a loop and press play. Each chord lasts one bar. Count four beats per chord out loud, then try strumming or humming your own rhythm over it. Notes tell you what; bars tell you when.