This function switches to another "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 meta1-[ META5_CHARACTER
binds this function to ^[[. You may then bind meta key
sequences with either of the two following formats:
bind ^[[A BACKWARD_HISTORY
or
bind META5-A BACKWARD_HISTORY
Works just like the META1_CHARACTER and the META2_CHARACTER but there
is no default binding.