Keys: where home is
Stay inside one scale for a while and something happens that no single note causes: one of the seven starts to feel like *home*. Melodies leave it and come back to it; endings on it feel finished; endings off it feel suspended in the air. That gravity field is a key. "In C major" does not mean "uses only white notes"; it means C is where the gravity points.
The itch
Two versions of the same melody. They differ in exactly one note, the last one. Play "Ends home" first, then "Ends one row short," and pay attention to your own reaction as the loop restarts:
That itch on the second version is not taste, and you did not learn it in a classroom. The melody spent the whole bar teaching your ear that C is home, and then it stopped one semitone short, on the note theorists call the leading tone precisely because of the way it leads. Your ear finishes the melody whether or not the music does.
Gravity, not a rulebook
A key assigns every note a job, defined by its distance from home. The tonic is rest. The note a fifth above is the strong counterweight. The leading tone is the itch you just felt. Nothing about this is a rule you follow; it is a field you work in, the way a dancer works in gravity. Tension in tonal music is, at bottom, distance from home made audible, and a melody or chord progression is a trip: how far out it goes, how long it stays, which road it takes back.
Portable homes
Like the scale it comes from, a key is a pattern, not a set of pitches: move home from C to F and every job moves with it, which is why a song transposed for a singer's range is still recognizably the same song. What changes between keys is not the internal logic but the address, and how far apart two addresses are, why C and G are neighbors while C and F# are strangers, turns out to have an exact geometric answer. That map is next.
In your music
Live carries a key setting per set and per clip, and when Scale Mode is on the Theory Aide extension treats that as ground truth: every chord it explains and every clash it flags is measured against Live's declared home. When the key is not set, the extension infers one from the notes, the same way your ear just did, by noticing which note the music treats as the floor.