These are escape sequences for non-printing characters. Well, the first two are. '\9' would most likely cause a compile error, since that format '\<digit>' is only defined for octal numbers, ie. digits 0-7.

'\0' means the NUL character, the value 0. Most of the time in C, this character is a special terminator value at the end of strings.

'\n' means the newline character, aka the line feed character, ascii value 10, which marks where one line ends and the next begins.