Can someone explain to me why this is a brilliant move?

I can't, I am not brilliant. I only know how to fry ants with a magnifying glass.

A very obvious and normal move, but you could have captured with the b pawn. So dxc6 much better than bxc6; for chess com parameters that are not usually that demanding, that was brilliant.

There are only two good options and the engine evaluates bxc6 and dxc6 to be approximately the same. I don't see any human justification for why dxc6 should be considered Brilliant. Perhaps it under-evaluates at shorter search depths. I'd just put it down to a quirk of the algorithm unless someone with better analysis skills than me can offer a better explanation.

hiiiiiii

You were already ahead by two rooks, so you were easily winning. 

You had no way to save the knight on a8, but it's possible that moving the pawn made losing the knight seem like a sacrifice to the computer.

Brilliant moves, according to the current computer definition, involve some sort of sacrifice. 

I see that dxc6 isn't in the list of the computer's top 5 moves.

It's not in the Cloud engine's Max analysis choice either.  (The top line, which showed that ..d5 was the engines top choice at that Max analysis level.)

I think it is possible any of the other moves would have triggered the Brilliant!!  mark too.

/ https://www.chess.com/analysis/game/live/60406278303?tab=analysis

It depends at what depth the engine calculates, the default is 18 depth, you can also make it go to 30 depth. If it is not the best move, it almost never or never evaluates a move as brilliant.

Well, that can't be true. As you can see,13.. dxc6 wasn't one of the top 5 engine choices but it was called "Brilliant".   

I re-ran it at 18 depth and it does not appear as one of the top 3 choices.

You can see the review at 18 depth although that could be 17 or so, it's not real 18 depth.
It also depends on the engine, it could be an older version of komodo.