system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: When musicians say a piece is in a certain key, what do they mean?
gpt
The key is where "home" is.  Sing "row row row your boat"--that note you started and ended on is the bottom of the scale that that song uses and is called the *tonic*.  If that note is a C, then you just sang it in the key of C major.  You could sing the whole thing starting on a different note, and it would be the same song in a different key.