It's never the notes that you're playing that sounds good, it's the notes you ain't.

Seen these first handful of comments and ain't a lot of ELI5ing here yet, so here goes.

Here is a random number pattern example: There is no real difference between me saying 1-3-5-6 and me saying 2-4-6-7 (both start with a number, jump two, jump two, and then jump one)

The gaps between both sequences are the same. I'm counting the same sequence of gaps, regardless of whatever number I start with. That's what keys are.

Also, yes chords are groups of "numbers" we'll call them still, but now we are saying all at once instead of in a sequence. If "1-3-5" sounds good played at once, then "2-4-6" will too. The distances between the numbers are what's important, not the numbers themselves.

Imagine starting the count at 1 is A, starting at 2 is B etc etc.