在 Python 字符串“ xaa”中，前导的“ x”是什么意思

That's unicode character escaping. See "Unicode Constructors" on PEP 100

The leading \x escape sequence means the next two characters are interpreted as hex digits for the character code, so \xaa equals chr(0xaa), i.e., chr(16 * 10 + 10) -- a small raised lowercase 'a' character.

Escape sequences are documented in a short table here in the Python docs.