A banner is one of the three battle standards recognized by Valyria, the others being the pennon and the colours. A banner corresponds with a company sized troop corps, and identifies its leader as a lord. Banners are usually entrusted to the care of cornets, which are gentle officers of the penultimate rank.
It is not known, but expected and hoped for that battle standards will be present in Chronicles of Elyria. Regardless of if banner pixels are present in game, anyone who marshals troops under a banner is a banner lord.