Experience Point. XP is a unit of measurement in FIFA and FIFA Mobile to quantify a user's progression through the game. XPs are generally awarded for the completion of matches, winning matches and titles, reaching your objectives & achievements, winning your favorite club's player bids in the transfer market, starting FIFA first time in a day and etc.
FIFA and FIFA Ultimate Team
In FIFA, having more XP will allow you to level up and to unlock some features such as the locked items on EAFC Catalogue.
In FIFA Mobile, gaining more XP will help you level up so you can unlock more Tactics and widen your Stamina capacity.
Playing any game mode in FIFA, FUT and FIFA Mobile my earn you some amounts of XP. By winning the matches, tournaments and Division titles, levelling up the game, winning the bids at FUT Transfer Market for players who are in your favourite clubs, you may earn even more XPs.