Calling writeln without any parameter merely completes the current line, or if nothing has been written, outputs a blank line.
Lines that start with // are called comment lines or briefly comments. A comment is not a part of the program code in the sense that it doesn't affect the behavior of the program. Its only purpose is to explain what the code does in that particular section of the program. The audience of a comment is anybody who may be reading the program code later, including the programmer who wrote the comment in the first place.