All music has rhythm; not all music has meter. Rhythm is the temporal pattern of sounds and silences in music; meter is the grouping of beats into recurring units the ear and body learn to anticipate. The simplest example is a 4/4 measure — four beats per bar, the first beat strong, the third less strong, the second and fourth weak. The ear learns this pattern within a few measures and predicts it; deviation from the prediction produces syncopation, a felt-but-not-played beat or an emphasis on a weak position instead of a strong one. The pattern of expectation and deviation is what makes music move — what makes a body sway, dance, or feel propelled forward. A piece of music that exactly matched expectation would be unbearable; one that violated it constantly would not feel like music at all. Rhythm sits in the productive middle, and the technical vocabulary of meter is the language for describing how composers and performers manipulate that middle.
The technical apparatus is small. Tempo is beats per minute (slow ≈ 60 BPM, walking ≈ 90–120, driving ≈ 140+). Meter is how beats are grouped — simple (2/4, 3/4, 4/4), compound with each beat divided in three (6/8, 9/8, 12/8), or asymmetric (5/4, 7/8, 11/8). Subdivision is how each beat is internally split; syncopation places emphasis where the prediction did not; polyrhythm runs two or more rhythmic patterns at once, the famous 3 against 2 of West African and Cuban music being the canonical case. What is more interesting than the vocabulary is what the vocabulary describes. Rhythm is processed in the brain by motor regions — the premotor cortex, supplementary motor area, cerebellum — areas that would be active if the listener were physically moving. Beat induction, the perception of a regular pulse in music that may not have one explicitly stated, happens automatically; even infants do it. Predictive timing — the ability to anticipate the next beat finely enough to clap on it — appears to be a specifically human capacity in its strongest form. Most species cannot synchronize to an external beat at all, and the few that can — songbirds, parrots, cetaceans, humans — share the trait of being vocal learners. Music, in this picture, is not a thing the brain perceives. It is something the brain does, recruiting the same machinery it uses to plan a step.
Hip-hop, drum & bass, trap (with its triplet hi-hats running against a duple kick), math rock with its asymmetric meters — contemporary popular music has dramatically expanded the rhythmic vocabulary that mass listeners are exposed to over the last fifty years, largely by drawing on cross-cultural traditions Western art music had previously left to specialists. AI music generation tends to default to common meters because the training data does, and extending into rhythmic territory the training data underrepresents is a current open research problem. The most striking present-tense connection is therapeutic: rhythmic auditory stimulation reliably improves gait in Parkinson's patients, because when the predictive-timing system the basal ganglia normally drive is failing, an external beat can substitute for the internal one. Rhythm is, in that clinical setting, the body's lost prediction returned to it through the ear.