Generally, the #pragma directives are intended for implementing compiler-specific preprocessor instructions. They are not standardized, so you shouldn't rely on them too heavily.
In this case, #pragma once's purpose is to replace the include guards that you use in header files to avoid multiple inclusion. It works a little faster on the compilers that support it, so it may reduce the compilation time on large projects with a lot of header files that are #include'ed frequently.
