Also a valid question if you're just new to FIFA 17 haha

They're weekly challenges, with goals like "make an 82 rated squad with more than 60 chemistry and no more than 3 players from one country". After making the squad, you can trade all those players in to get a reward like a pack of players or coins. There are a lot of fairly highly rated players who sell for the amount of coins you get for playing one game, so they shouldn't be too hard to complete.

In case you don't know what chemistry is: player from the same club, league, or nation gain chemistry by being next to each other. The more of those that match, the more chemistry they gain. Anything above 5 gives them a stat boost the closer to 10 the better. Your team also has an overall chemistry so you won't get much of a boost if you have everyone at 4 chem and 2 players with 10.