Write /* to start a comment and */ to end it. Everything between is ignored by the compiler, even if it spans multiple lines of code. Multi-line comments work well for long explanations or temporarily disabling big blocks of code.
Wrapping many lines in /* */ is easier than adding // to each line individually. Be careful not to nest multi-line comments. The compiler ends at the first */ it sees. A second */ causes a syntax error because it has no matching start.