What exactly is the L prefix in C++?

I understand what it does: specifies a string literal as a const wchar_t * (wide character string) instead of const char * (plain old characters), but how is it actually defined?

Is it a macro of some sort? Is it an operator for GCC compilers? What is it?

The literal prefixes are a part of the core language, much like the suffixes:

'a'    // type: char
L'a'   // type: wchar_t

"a"    // type: char[2]
L"a"   // type: wchar_t[2]
U"a"   // type: char32_t[2]

1      // type: int
1U     // type: unsigned int

0.5    // type: double
0.5f   // type: float
0.5L   // type: long double

Note that wchar_t has nothing to do with Unicode. Here is an extended rant of mine on the topic.

It's called an encoding prefix:

2.14.5 String literals [lex.string]

string-literal:
| encoding-prefix_opt" s-char-sequenceopt"
| encoding-prefix_optR raw-string
encoding-prefix:
| u8
| u
| U
| L

and marks a wide string literal:

11) A string literal that begins with L, such as L"asdf", is a wide string literal. A wide string literal has type “array of n const wchar_t”, where n is the size of the string as defined below; it has static storage duration and is initialized with the given characters.

The meaning of L here is wide character: wchar_t. String with L is coded in 16bit rather than 8bit, take an example: