Not understanding Best Move.

Im terrible at chess but I dont understand why a3 is a good move. Does this not open the file for his rook to trade the pawn and threaten the bishop and K side pawns?

Taking a pawn is usually good: it will queen a3-a2-a1=Q

If you take the pawn on a3 your opponent cannot capture the pawn on B7. If they do that you play A2 and promotion is unstoppable. Even giving away the bishop doesn't matter. You promote with check.

Once the rook moves to A1 and is tied down to that pawn, you can go about your business of trapping that Bishop. Which looks like what you were trying to do but you were going about it before solving the problems on the a-file . 

This is a perfect explanation thank you.

Obviously you're doing a game review which is great. You may or may not be aware that when you're in that review you can still make hypothetical moves and see what the computer says about them. If you are looking at a move that you think someone would play but the computer doesn't even talk about, very likely it's really bad. You can manually make that move and the computer will reevaluate and tell you how bad it is and why.

The whole point of Chess is to be able to see farther ahead than your opponent. As you get better, obviously you will be able to see more stuff. The more you analyze the more of these questions you will know the answer to. If the computer says there's only a slight difference between two moves, for now don't worry about that. But if there's a fairly large difference, try to figure out why because that is showing you a situation where you're thinking can be improved.