The idea is to simply remove the dark-squared Black bishop which attacks both the b2 and f2 squares. Instead, 6. c3? is not a good move, allowing 6...Bxf2+.
I am not very used to the design of your pieces. The bishops look like knights to some degree. If I wasn't careful, I would be looking at 6...Nxe5.
I just played this blistering game (see above) - and I was going through the analysis afterwards when I was stumped to see 6. Nxd4 recommended. The computer takes you down a game tree that leaves you in a terrible position, and the following game trees recommended to black all seem to lead to White's advantage. On the other hand 6. c3 turns out very well for white.
Why does the computer recommend such a poor course of action?