9.6 Why does the Backspace key invoke help?

The Backspace key (on most keyboards) generates ASCII code 8. C-h sends the same code. In Emacs by default C-h invokes help-command. This is intended to be easy to remember since the first letter of ‘help’ is ‘h’. The easiest solution to this problem is to use C-h (and Backspace) for help and DEL (the Delete key) for deleting the previous character.

For many people this solution may be problematic:

When Emacs runs on a windowed display, it binds the Delete key to a command which deletes the character at point, to make Emacs more consistent with keyboard operation on these systems.

For more information about troubleshooting this problem, see If DEL Fails to Delete in The GNU Emacs Manual.