Why is this a great move?

I'm reviewing Hikaru vs Levy here, and at move 12. Bf4, the engine considers this a great move, why? I've followed the moves the engine provides to demonstrate this and arrive at a position that doesn't feel like it is massively in white's favour (at least not simply because the bishop is there).

Can someone explain in simple chess-principle terms why this move is so strong?

A great move means that this is the only move in a position, that works. That means, that this is the only move that doesn't lose (if a position is drawish), and the only move that keeps your advantage (if a position is winning for you). You can also see that in self-analysis, that that move is the only one. Although, sometimes it happens, that the great move is not the only one that works

For example, you're in check, and one move to evade the check leads to a forced checkmate in x moves, another one leads to losing of material, and only one keeps the position drawish. That is a great move

I see what you're saying, thanks!

I'm definitely no GM, but I'll take a stab at why this is good:

Bf4 is the only move that safely adds another defender to the pawn, meaning an exchange of pieces is no longer viable for Rozman.

Don't quote me on that. I think that's right, but don't.

In Game Review, "great move" means "the only move that's good".

It can be the most banal, obvious move (like here), it doesn't matter.