The #include I believe is accessing code created available from source online. where and whose it is, that is beyond me.
This code is allowing us to express strings as it checks for them and closes when there is nothing left. There are no while/else/if statements, it does not have a reason to stay open and check for conditions, so it closes.
the main function is declaring the following indented code as the primary function as an executable within the code.
the expressions are not recognized as rules set into the code, rather than just interpretable information for people to read that gets put into code, such as strings.
declaration statements define the rules within the main function.
return statement sends all the information confined within the main function and executes it.