Keys! is a control panel/system extension that adds command key shortcuts to dialog items in modal dialog boxes (buttons, radio buttons, and check boxes). It underlines a letter (generally the first one, as far as possible) of the name of each button or check box; if you hold down the command key and type that letter, it will be as if you had clicked on the button.
If the dialog box doesn't contain an editable text field (e.g. most alert boxes), you needn't even hold down the command key - just type the letter. However, scrollable lists count as text fields in this regard; for example, in the standard open dialog box there isn't any text field, but you still have to hold down the command key there because you want to use the plain letter keys to select filenames.
The letters C, V, and X are never used, because they are already assigned to the standard editing commands, which must be available in dialog boxes. This means of course that if a button is named "C" (like in the preferences dialog of Think Reference), it won't be assigned any shortcut. Sorry. On the other hand this assures that the cancel button is almost always command-A, regardless of whether it is named Cancel or Abort or Abbrechen...
Kurth, Stefan. (1995). About "Keys!". Electronic Document.