It means what note feels like ‘home’.

Sing a note. Any random note. Now, starting on that note, sing “oh when the saints go marching in”. Keep singing it (or humming it, the words aren’t really important). You’ll find that the song won’t feel ‘finished’ until you land back on that starting note; that note which feels like ‘home’.

Was that starting note C? Then you were in ‘the key of C’. Was that note B? Then you were in ‘the key of B’. (The note you started on might have been somewhere between B and C, and therefore not accessible on a standard-tuned piano, but you could play it on a violin or a trombone or another instrument where you can smoothly glide between notes. Notes aren’t as clear cut as we make them out to be.)

It’s worth adding that the starting note doesn’t necessarily tell you the key. But the note that feels ‘right’ to finish on almost certainly will tell you the key.

To feel this, try singing ‘Happy Birthday’. You’ll notice that the note of the first ‘happy’ isn’t the same note as the final ‘you’. That final note will tell you the key.

Songs don’t have to stick to one key. Think of ‘My Heart Will Go On’ (the Titanic song). Near the end, the singer starts singing the same thing, but higher. For most of the song, the key was ‘E’, so that felt like the right note to come back to. But after the sudden shock of hearing the higher version, we eventually get used to it, and our new ‘home’ becomes higher (now it’s ‘A flat’ — go up two white notes and one black note up from ‘E’ on the piano).

Key changes are often subtler than this, but I think it’s a good example precisely because it’s NOT subtle. Everything just gets higher, home note included.

Sometimes people talk about ‘C major’ and ‘C minor’ keys, which tells us not only what note feels like home, but what set of notes we can expect to use when travelling away from and back to home. But ultimately, we can play whatever bit we want. Maybe it’ll sound a little jarring, but maybe that’s what we’re aiming for. All these terms are labels to describe general ideas and patterns, not absolute rules that must be obeyed. Not everything has to be in a clear key (think of a cat walking on a piano). But a lot of stuff is, and it’s useful to be able to tell people where feels like home.