If you mean that "file.L" is actually inside quotes that means that it is a string literal. How it is used by the program depends on where it's used.
#include "file.L" This includes the file.L file into the source file. file.L have to contain C/C++ code or otherwise it will not compile.
It could also have been used in connection with std::ifstream, std::ofstream or std::fstream. In that case it is naming the file name to read to/write from.