What does "quoting metacharacters" mean?

You will find that people also use the term 'escape', which can be equally baffling.
Some characters have special meanings, as you are aware in regular expressions these are called 'meta-characters'. Within Perl (and languages such as UNIX shells) placing single quotes around a character removes its special meaning, such as '*'. For single characters this is rather tedious, so placing a back-slash \ character in-front has the same effect, such as \*. The Perl Q relieves the tedium further by placing a \ in front of each non-alphanumeric (includes _) character for us.

In the olden days we sometimes needed character pairs which were prefixed with the <ESC> character, and this is how it was done, hence the term 'escape'.
You will note that the \ character removes the special meaning from a special (meta) character, but can also add a special meaning to an ordinary character, for example "\n". It has to be inside double-quotes or qq (interpolation) for that to happen.