this used to be only for colemak and qwerty, so the explanation only fits those two. you can extend it to dvorak tho so it's fine
the keyboard layout colemak shifts the keys around into a different location than they would be on a traditional qwerty keyboard. however, if you're typing with the colemak layout on but you press the keys in the location they would be in a qwerty layout, you end up with a bunch of gibberish. this tool just lets you decipher (and create) whatever colemak-qwerty gibberish you want.
you can even convert it several times and use it as a simple cipher.
eventually, it will go in a cycle.
here are the cycle amounts:
qwerty <> colemak: 42
qwerty <> dvorak: 210
colemak <> dvorak: 24
also this only supports main keyboard stuff so if you have other symbols on your numpad that don't change, this might change it