Music theory is the vocabulary for what you already hear: names for the patterns that make melodies, chords and rhythms work.
Music theory is a set of names for things your ears already know. You can hear that two songs feel similar; theory gives you the words to say why: same chord pattern, same scale, same rhythm.
It is not a rulebook. Nobody gets arrested for breaking it. It is closer to a map: it describes where music usually goes, so you can find your way faster.
Music theory describes how music is organized: how the 12 notes group into scales and keys, how notes stack into chords, how chords chain into progressions, and how time divides into beats and meters.
The practical payoff is speed and communication. With theory, you can learn songs faster because you recognize the pattern instead of memorizing every note, and you can tell another musician "it is a ii-V-I in G" instead of humming for five minutes.
Theory works best paired with ear training: the names only matter when they are attached to sounds you can actually recognize.
Formally, Western theory covers pitch organization (intervals, scales, modes, keys and their relationships), harmony (chord construction, function, voice leading), rhythm and meter, and form. Each layer describes conventions extracted from centuries of practice, not laws.
Descriptive, not prescriptive, is the key framing: theory summarizes what musicians have done and why it tends to work on the ear. Composers break the conventions constantly, and the breaks work because the listener carries the convention as an expectation.
A useful learning order: intervals first (the atoms), then scales and keys (the palettes), then chords and progressions (the sentences), with rhythm running in parallel. That is the order the Tavi La Si curriculum follows.