To take it a bit further, “Do” is a single note as you said. “Do Re Mi Fa So La Si Do” refers to the difference between the notes (eg jump two, jump two, jump one). And that pattern of differences is called a key.
You can start anywhere you want though and decide that note is “Do”. If you start on C and play that pattern, you are in the key of C. If you start on a D and play the same pattern, you are in the key of D.
crepuscular-tree t1_jdbajyq wrote
Reply to comment by PhatLittleGirlfriend in ELI5: How can songs be in a certain key? (And a few more questions about music) by Glum-Airport-4701
This is a great answer!
To take it a bit further, “Do” is a single note as you said. “Do Re Mi Fa So La Si Do” refers to the difference between the notes (eg jump two, jump two, jump one). And that pattern of differences is called a key.
You can start anywhere you want though and decide that note is “Do”. If you start on C and play that pattern, you are in the key of C. If you start on a D and play the same pattern, you are in the key of D.