What is the idea behind Fischer's 10. Na5 move in the RLE?

The knight has nothing to do on b3. The idea is to make the transition Nb3 --> Na5 --> Nc4. However, black prevented it with 10...b5. Btw it weakened the c5-pawn. Something like that.

The queenside pawn weaknesses and Black's inability to find a safe place to tuck his king and get organized are major opening problems for the second player. The mainline wound up to be 9...Bg4 10. f3 Be6 11. Bf4 (Now 11. Na5 can be met with 11...b6 because the c4 square is covered) 11...c4 12. Nd4  (12. Na5 b6 13. Nc6 Bc5+ 14. Kf1 Bd7 15. Nd4 O-O-O) 12... O-O-O 13. Nc3 Bf7.  In the mainline black has untangled himself and got his king out of the center.

Lines in Panczyk and Ilczuk's book (some, not all).

and since Black played b5  now Nc6 is an option,  ironically at White's choice because the N on a5 can't be kicked by a pawn.