Can someone help me understand why a move here is so touted by the engine?

It looks to me that you can win material by planning to double your rooks on the a file eg Ra4 planning Rfa1. Black has very few moves, Ra8 can't move, Bc8 can't move, Na6 can't move, the king is stuck, the Rh8 doesn't play. It's a winning position for white, just compare the great position of each of your pieces to the miserable position of their counterparts to confirm this. And your comment on the opposite bishops is only partially true. If you get to a simplified endgame eg bishops and pawns then yes it is very drawish, but if one side is attacking in the middlegame with other pieces on the board it is a great advantage to the attacker because they dominate squares of one colour and the defender cannot challenge this.

And also aside from the dominant position, yes white is down a pawn but the extra pawn is doubled meaning it is worth very little. Don't be too worried about your material situation, it's all about the position and the power of the pieces you have. The position you posted is a great example of this.

 Not a very high elo player myself, but at first glance i just love your position. For starters you have a great bishop cemented on d6. Black's knight, which was quite silly on h6 to begin with, is pinned to the rook on h8. Consequently the defence it is giving the pawn on b4 is just an illusion. All your pieces are out most of black's pieces haven't even moved.

What is even worse is they CANNOT Move. He can't castle. He can't move the bishop without hanging it. He cant meaningfully move the rook on h8 without hanging the knight.

Plus he has another set of doubled unprotected pawns on the f file.

Black's position is all round miserable he has just no moves. You can play Ra2 followed by Rfa1 and just be a whole piece up, while his pieces still cannot really do anything. Its a lost position for black.

The only chink in white's position is that pawn on g3, which sort of gives a window to your king a little bit. But positionally white could not be better.

Man, I love this chess community.  Thanks for the comments, y'all.  It's really nice to be able to get advice from other players and I'm routinely shocked at the generosity of other players who take time to look at these sort of posts. 8) 

I felt like the position at this point was winning for white.  I saw how black had no moves available.  I still didn't convert this to a win b/c of I need to stop using an engine so quickly to analyze my games and start doing it myself and afterwards check the engine to confirm.  When I did go over this in the engine, it showed a huge advantage for white and some really weird moves recommended including sacrificing a rook down some of the lines.  It had some real computer moves that I'd never try in a real game.

I had a plan of limiting his mobility earlier and it worked, I saw it worked, I didn't realize how well it worked though.  It's a shame I lost the game.  I am going to start from this position vs the computer and see if I can figure out how to win this one.  I am still a bit sketchy on how to convert this to a win.

Chess is a cruel teacher.

A few points :

• Chess is a team game - your bishop on d6 means he can't use half his team (Bishop on c8 is now very very sad, rook on h8 struggles too)
• Pin on a- file looks strong
• Forget "opposite coloured bishops means draw" unless it's an endgame (and even then only if it's just the bishops is it really drawish). 
• Instead opposite coloured bishops means positional stuff matters more than material, and the side that gets an attack often wins very quickly as the other bishop can't defend half the squares.

No, you've got to be careful.  1.Ra4 is better than 1.Ra5.  After 1.Ra5, Black can play 1...Nc7 and since the Rook is hanging on a5, White must trade on a8.  Black's knight sucks, but he hasn't lost material.  White is still better, but the move 1.Ra5 is not best.

After 1.Ra4, the Rook is protected by the White Knight, and so 1...Nc7 is unplayable as 2.Bxc7 just wins a piece since 2...Rxa4 white can recapture with the Knight.

White is by all means winning, but you still have to be careful about the subtle differences between moves like 1.Ra4 and 1.Ra5.  The former will outright win material after White doubles up on the a-file.

I gotta work on my analysis.

This is a common misconception. If you can trade off ALL the other pieces (except kings of course) then it become drawish. BUT if there are other pieces, then opposite colored bishops favors the attacker, because they are up a piece on the color of their bishop. In your game, you can see your bishop is a monster on d6, while your opponent's bishop on c8 is a Tall-Pawn.

Yeah, that's true, I somehow failed to realize that once you take the Knight, the Bishop suddenly protects the Rook.  Stupid me!

Stockfish 8 20-ply only rates the move 21 Nc5 as +0.46 for white, both through Lucas Chess and by clicking on "analyze" on your game board in your post here.

Stockfish really likes 21 Ra5 (+2.55), surely because of the threats Rfa1 and Nc5.