//

Denotes the following line is a comment that is ignored by a compiler. I know for sure it is a C++ thing; I'm not too sure of other programming languages. Example:

// ATM Program
// This program simulates an ATM working environment.

This is a comment that would explain to a person reading the source code that it was a program that simulates an ATM. Just an example. Comments can really say or describe anything.

In a URL, // denotes the beginning of a hostname.
The next group of characters, up to but not including the next / character, should be the name or IP address of a computer.

As previously noted the // is used indicate a comment in C++.

Interestingly, a common assertion about the C programming language is that the compiler ignores all whitespace in your code, so that if you really wanted to, you could type an entire programme onto a single line (see: obfuscated C). This is not entirely true because the "newstyle" comment found in C++ (i.e. //) uses the end of line (EOL) to terminate your statement. Now isn't that interesting?

Hopefully you'll be able to use this information to your advantage to grift some 133t h4X0r out of some hard earned cash.