If you want to indicate the folder where it'll be saved, the tutorials give the example of using
("user://"), which will vary according to the OS.
Personally, I prefer using
(./), as this defines the folder where the executable is. So,
./saves would refer to the saves folder wherever the executable is, whether it is
/home/bla/game (for Linux) or
If you plan to have the game as HTML5, do use
(user://), which will save to their browser's cache. Similar question here - https://godotengine.org/qa/30304/how-can-create-save-game-system-that-works-game-exported-html5