[sls]

C-h ? will show you help on help, among them:

C-h a - apropos

C-h k <key sequence> - detailed docs on the command run by key sequence

C-h m - docs on current minor modes and major mode

C-h w <command> - what keystrokes will run command

There's a lot more as well.

[timonoko]

C-h is backspace and also "move cursor left" on many ancient video terminals. So it was always remapped.