I can not understand the following para in Emacs Lisp Reference :

In a string, the 2**7 bit attached to an ASCII character indicates a
> meta character; thus, the meta characters that can fit in a string have
> codes in the range from 128 to 255, and are the meta versions of the
> ordinary ASCII characters.  (In Emacs versions 18 and older, this
> convention was used for characters outside of strings as well.)


One bit has two states.
Does 2**7 bit denote 7 bits ?

What does `thus, the meta characters that can fit in a string have
codes in the range from 128 to 255, and are the meta versions of the
ordinary ASCII characters. ' mean ?

Thanks