Iron Banner is a monthly endgame PvP activity at Crucible arena. There is no rating. It is just you, your team, and your enemies. The game mode in Iron Banner calls Rift - Each team has a Rift at their base. A Spark spawns in a central location on the map. A Guardian must pick up the Spark and take it to the enemy team's base to destroy their Rift.
However, this game mode is also a source of really good rewards, including meta legendary weapons and special armor sets. Buy Iron Banner Boost by Overgear to overcome any problems with this activity, complete bounties, unlock armor sets, and earn other good rewards.