In spite of all my efforts, I still don't really understand keys...

I have to play a song which goes D A G D G D Em7 A7 ~ simple simple, I know, BUT....BUT!! on the piece of paper where the chords are written it also says:
Original Key: F Capo: 3 Play: D

What's that supposed to mean?
I don't get the "Original Key: F and Play: D" part.

Could somebody please explain this to me before I get even more confused?!
I assume you know what a capo is right?

When you put a capo on your 3rd fret, all of the chords go up three semi-tones. This means that when you play D formation, you are playing F. (D up 3 semitones is: D, D#, E, F).

The song is in F, and it will seem as though it's in D because D is the main chord 'formation' you are using.
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