A meta character is a character typed with the META modifier key. The integer that represents such a character has the 2**27 bit set. We use high bits for this and other modifiers to make possible a wide range of basic character codes.
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. See Strings of Events, for details about META-handling in strings.
The read syntax for meta characters uses ‘\M-’. For example, ‘?\M-A’ stands for M-A. You can use ‘\M-’ together with octal character codes (see below), with ‘\C-’, or with any other syntax for a character. Thus, you can write M-A as ‘?\M-A’, or as ‘?\M-\101’. Likewise, you can write C-M-b as ‘?\M-\C-b’, ‘?\C-\M-b’, or ‘?\M-\002’.
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https://www.gnu.org/software/emacs/manual/html_node/elisp/Meta_002dChar-Syntax.html