This is the main theme of the kusubooru imageboard. Apart from that this tag is given to any picture that portraits actual ticking. This tag should be paired with other tickling special tags like m/f, f/m, many/f, etc. that show who tickles who. It's also advised to add the parts of the body that are tickled.
So if a picture portraits a male who tickles a female on her feet, it should have the tags: tickling, m/f and feet. Simple huh?
If an image does not portrait actual tickling but it is implied that it's going to happen then the tag you_gonna_get_tickled should be used instead.
See howto:tag for more details.