Adding one more point to @betlista’s answer.
You might have / would come across
//something, maybe take input from some file
These are for testing purposes, so that you store the sample test cases in a file and you run your code to see the output without having to type the sample inputs again and again during debugging.
You can also use redirection.
After compilation, with the executable file if you write
:~$ a.out < test.in > test.out
It will take input from test.in file and will write the output to test.out
You can ignore these here in the beginning, but you must learn these as some contests do ask for the output file e.g. GCJ