In the beginning there was cacophony, and it was good. But not everybody thought so.
Several hundred years ago, Christian monks discovered how to play several notes simultaneously in a way that, to them, sounded like Heaven.
Choose a scale and select every other note of it until you have 3 notes (notes 1,3,5)
Play these 3 notes at the same time and that’s a chord.
If you add the next note using the “every other” pattern (the 7th) then you have 7th chord. Keep playing the scale higher and you’ll come across the 9th, 11th, and 13th notes, giving you the 9th, 11th, and 13th chords.
Now you know everything about music. Go outside and play.