Ultimate Team is a game mode whcih allows you to build "All-Star" squads to play with online against other people.
I have never myself played FUT, but I know some people who do, and they think it's absolutley amazing. You can care for your own team, and with some luck even achieve the very best players in the world, like Messi, Ronaldo, Neymar ect. In this game mode, you are able to acquire players from various teams to build your own squad. You can then take this team and play one on one games or special tournaments online or against the computer.
The way you acquire players is through packs (like baseball,football cards) and it works similar by trading on Ebay like auction system. You get money for the packs by playing in games or trading your existing players on this auction system. There's a little more to it than this, but generally If you enjoy collecting sports cards, the stock market and/or trying to manage a team by acquiring various players to meet your needs you'll really enjoy it. the add on is free. It sometimes takes a while to get started, build your team from the ground up and learn all the little managing specifics (acquiring players, using contracts, formations, etc) but once you get used to it it's very addicting.