SpotPass is a type of wireless communication that allows 3DS users to download content via the Internet, for free. As seen with games such as Dead or Alive Dimensions, Kid Icarus: Uprising, and New Super Mario Bros. 2, said content is not really new, but rather is content that is already developed or has a basis of data that is already established in accordance to the game and is able to be modified. The same holds true with Fire Emblem Awakening. Fire Emblem Awakening offers a ton of SpotPass content.
Upon the release of the Japanese version of the game, Nintendo began distributing SpotPass teams. There are now, as originally announced, 120 different SpotPass teams, which are organized into 12 groups of 10 teams, and the 12 groups are all (except for one) themed after a particular Fire Emblem game. Each of the 10 teams of the 12 groups consist of one particular legacy character from the past games, who leads a group of generics, and may be interacted with in pretty much the same manner as you would with an Avatar that you meet through StreetPass. Each group is initiated to appear manually via what's known as a Download Box, once Nintendo releases that group for distribution, with each team appearing on their own one after the other for the next couple of days after you summon that group. This means that, over the months after the game came out, Nintendo let you get 120 returning characters to recruit.
At the same time as when Nintendo began releasing SpotPass teams, they also started to distribute unique Dual Tag teams for the player to face off against in Dual Tag, the game's local multiplayer mode. These new SpotPass challenges are generally much more difficult than the ones initially available, but completing them nets you better rewards, including powerful weapons and Fame points, the latter which is used to earn even more items.
Not long after, Nintendo would also begin offering powerful weapons directly to the player via the Download Box. There are twelve in total, including Luna (FE2), Tyrfing (FE4), and the Double Bow (FE10). Once they're available, you take it for use in your save file.
In addition, over the months after the release of the Japanese version of the game, Nintendo "unlocked" the ability to access a number of exclusive SpotPass sidequest chapters that relate to the game's story and let you recruit specific characters who you couldn't before from them, with the only prerequisite being that you needed to reach a certain point in the game to reach them.