If you are referring to the picture that they have as their profile, its just the defult rune given to members who have not chosen a unique picture as their profile picture. If you're referring to the minature rune next to their name, its just an abbreviation of their rank.

If you're referring to the purple rune in the lower-right, it indicates Legendary rank. Legendary rank is reached once you reach the rank of Aedra and choose to reset.

I meant the purple rune, yes. Thanks guys :D