StreetPass is a feature available in the Nintendo 3DS. When enabled, it can allow two Nintendo 3DS systems to connect with each other and exchange data. While data is usually exchanged between a game feature called "StreetPass Mii Plaza", data can be exchanged for 3DS games, like Fire Emblem: Awakening.
In order to start a StreetPass, players must first create a StreetPass team, called My Team, consisting of 9 characters from their current roster and the Avatar, totaling 10 characters. These teams can be changed at any point and will update to your character's current Class, Stats, and Skills as you play. DLC and StreetPass characters can be put onto the StreetPass team (Except in the American release as there is curently a glitch that will send out a random team if you use downloadable map characters or items).
When a two players StreetPass exchanges data their teams will be exchanged. The team data received will appear on the Map as a Green Unit. Players can interact with these Units by buying an item in that Avatar's shop, scout the Avatar, or battle the Avatar's Team, similar to the function of SpotPass.
From the Avatar's shop, players can buy weapons and items that that Avatar's team is currently holding in their character inventories. This includes Forged Weapons which will be adjusted with the Forge costs. Legendary weapons such as Ragnell and Book of Naga will not be able to be sold in these shops.
Recruit allows the player to hire the other player's Avatar for a fee.The price of the Avatar is dependent on the current stats of that Avatar and the skills they have acquired. This can allow the player to make up for lost units in Classic Mode and will permanently stay in your party until they are defeated. You can re-recruit them if you StreetPass them again.
Battle obviously allows you to battle the other player's team. The teams are computer controlled and the difficulty of the battle is dependent on the other player's difficulty setting. They will not use the Pair Up function. If the player defeats the other player's My Team, the player can hire the other player's Avatar for free.