Meter refers to the underlying pattern of strong and weak beats that provides a living, dynamic time/energy “grid” for music. Musicians align notes and chords with the metric grid to produce a varied range of sensations, including (though not limited to) steadiness, agitation, scampering, propulsion, or repose.
To understand the power of musical meter, we can trace it back to its psychological and physiological roots. Strong and weak beats fall into recurring patterns in the same way that our feet do when we walk, or our jaws do when we chew. By “strong” and “weak,” we mean that certain beats feel “heavier” than “lighter” beats. And rather than counting beats in an uninterrupted series (1, 2, 3, 4, 5, … 17, 18, 19, … 215, 216), we “meter out” or “measure” these heavier and lighter beats into groups or “measures.”
Going further, the downbeat of any measure feels the heaviest, like the “starting point” or “foundation” for the beats that follow. And the last beat in any meter feels especially light, unbalanced, even “edgy.” And so we call this last beat an “upbeat” because of its need to “land” on the upcoming downbeat. All of the other beats within each measure possess varying degrees of upbeat or downbeat qualities.
A melody gets much of its expressive power by aligning certain notes with “strong” (or heavier) beats and other notes with “weak” (or lighter) ones.