A C++ style comment is two slashes (with no intervening space). It causes the rest of the line to be ignored. This kind of comment is often used because it requires less typing.
Quite often this kind of comment follows a statement on the same line as a point of explanation. For example:
return 5; // 5 means warning
The return is executable and the comment explains the return value.
This comment is new to C++, though C++ also accepts the older style C comment.