I'm late to the game but bringing my band geek knowledge to try and ELI5.

The white and black things you press on a piano are called keys, but that's not what it means when we talk about the KEY of a song. If you made a song using all of the strings on a guitar or all of the keys on a keyboard together randomly, it would not sound like MUSIC, it would just be noise.

If you make up a song and sing it to yourself, you could then go over to a piano and pick out what the notes are, and write them down. There will be a KEY that is most natural for that collection of notes. For instance, if the notes you wrote include a lot of B flats and E flats, your song's natural KEY is the B Flat Major Scale, which goes B flat, C, D, E flat, F, G, A, B flat. (When you are learning any instrument including voice, you will learn all the major and minor scales.)

So a song is just a series of notes with a set relationship between them. You could transpose the song you just wrote (change the notes, keep the relationship between notes the same) into any KEY, although it might change the mood of the song. Changing the key is useful for singers if the song in its original key has some high or low notes they can't hit. But if you want to play music with more than one person/instrument, you all need to be playing in the same KEY.