DF means Debris Field. If you see a 'T' next to a planet in the Galaxy view then it means there is Debris there. Hover your mouse over the T and it will say how much, you'll see 2 numbers. The first number is Metal, the 2nd is Crystal i.e. 10,300:21,000 would mean there is 10,300 metal and 21,000 crystal floating out there.

You can send recyclers to Debris Fields once you get them. A debris field is created when someone attacks a planet and the defence on that planet destroys some of the fleet.

So basically, if you have a big ass fleet and are attacking a well defended planet, there are going to be some losses. So you send your recyclers to arrive a few mins after your fleet to the DF to clean up and recover some of your lost resources.