The keymap debugger can be used to figure out what a key does. It is useful if you want to develop scripts. On the left, it displays the names of the currently active keymaps, which define what keys are active at this place When you press a key, the action it triggers will be displayed on the right.

To close the keymap debugger, press ShiftESC (a lone ESC will also work if it isn't bound by the active keymap).

Reference

Keys:
AltK from almost everywhere
See also:
Keymaps in script manual