I must be even before the title beginner. I don't know what STD, thread or chrono, is. What the #include means... and definitely not how to implement any of these..... and what do the semi colons mean?
Well the double colons express that you are accessing a certain namespace scope.
std is the name of the entire c++ standard library, all standard functions and classes are in this namespace. To access the standard library, we thus use std::
#include <x>
Is a "macro", the pre-compiler simply pastes the contents of file x into the code where you have stated "#include <x>".
thread and chrono are std libraries, including them into your code gives you access to use their functions and classes.