Something that suggests the presence or existence of a fact, condition, or quality. (TheFreedictionary)
I guess some of you have read your Barthes but let's not get too semiotic! In short the sign shows that something is going on. Even the most simple lifeforms are able to recognize and learn signs. Fire may burn you and fast approaching object may hurt you. But understanding some more complex significance requires a much more effort put into learning the possible meanings of signs.
And what does this sign mean? At least it means someone was there and wanted to make a statement. I leave it to you to find out what that statement is.