I think I have worked it out. In happens in PVP and I am AD. I think it signifies combat. I am going to assume for EP players it turns red and DC blue?
The aggro table can be understood as that enemy's "awareness." If an enemy that would attack you on sight becomes "aware" of you, you are on that table. If there are multiple people on that table--a common scenario in dungeons--the person who has the most aggro (or threat) is the one that is targeted first. Taunts temporarily place the user at the top, regardless of aggro values of each target on the table.
This is kind of "MMO 101" level stuff, though. You might find it worthwhile to look up some videos on the topic, not just for ESO either, but ESO based ones will obviously apply in a more 1 to 1 fashion.