This tag is for any time a character shows off a cherry stem that they have just finished tying into a knot using only their tongue. It is intended to be a display of (and testament to) their lingual dexterity, and suggests that they are very good at kissing... And, perhaps, other things as well.
Often done with the stem sitting on an outstretched tongue. It also may be accompanied by a cheeky, seductive, or even lewd look on the character's face.
Technically speaking, this tag is for any depiction of a cherry stem tied into a knot... But the only* time this really ever happens is when they just did it with their tongue, so it's sort of a moot point.