This function switches to an "alternate" keymap. That is, after this character is hit, the next key hit can have a new definition. This is how the Escape sequences are done. In fact, the escape key's default binding is META1_CHARACTER. For example:
bind ^X META2_CHARACTER
binds this function to ^X. You may then bind meta key
sequences with either of the two following formats:
bind ^X? HELP_CHARACTER
or
bind META2-? HELP_CHARACTER