It means that after 1.Rxf8 Qxf8, if White then plays 2.Qd8+ trying for a back rank mate, it doesn't work for White, since Black can play 2...Qxd8 3.Rxd8 Rxd8.
(That is, after the first move by each side, both sides now have two pieces covering the important d8 square.)
Could someone help me out with this I was reading through bobby fischer teaches chess as you can no doubt see off to the side and I went to a random page and did a position and in this one when the white rook at f1 takes the black rook at f8 there is a question in the book I had to answer it said which piece should I recapture the white rook with the black rook at g8 or the queen at g7 I chose the queen and the answer on the next page said I was correct because it would still be two against two what does that mean ?