I dont understand this common bishop sack.

https://www.chess.com/analysis/game/live/62521166029?tab=review
At move 10 they trade their bishop for a pawn, supposedly to weaken my pawn structure, but i feel like that is not a good trade, im not understanding why that is a good thing. Their sacking a bishop, well not always, they often retreat the bishop when threatened by the pawn as well, this user resigned suddenly im not sure why. I also have been playing the computers alot and they do it as well. What is the proper follow up for black after sacking the bishop like this. Usually im like, ok great, ill just castle on the other side then. I just see this happen alot but dont see the benefit. What am i not seeing?

What Bishop sac are you referring to?

bc3, they take the knight, and sac the bishop, this is regarded as best move by the computer. People at least in my rank do this ALOT. Its why im asking.

Im sorry i meant a3 bishop xc3+ or move 6.

Ah. Well here after Bxc3 bxc3 your pawn structure is damaged with the pawns on c2 and c3 doubled plus the pawn on a3 isolated. These are long term issues which won't go away and will probably cause more and more of an issue the longer the game goes on and the more pieces come off the board. But it's still early here, so is this plan good for black? It's hard to say, he could have played 5.Be7 or 5.Bd6 and gone down another path. After playing 5.Bb4 and your reply 6.a3 he is pretty much obliged to follow up with Bxc3+ because retreating would be wasting time.

Yea, that usually is how it goes when they retreat it, they waste two moves and advance my pawns so may as well take the knight, but why is it the appropriate response to 5.

It's not necessarily. For a beginner player it's just an obvious move to pin the Knight because pins are good right? But here you commit to exchanging the Bishop for a Knight which is not always a good idea. Anyway I wouldn't worry too much about the engine evaluation, the game is at such an early stage that other things will happen to decide the game.

Well, you were asking why black sacrificed the bishop for a pawn on move 10. Later it turned out that it was not move 10, not a sacrifice and not for a pawn.

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Firstly, it would only be a sacrifice if he took say a pawn for the bishop. He took the knight and if you know piece values, you know knights and bishops have the same value. In doing so he forced you to take back with the pawn, damaging your queen side pawn structure. As it's still early in the game, it'll be difficult to see whether these weaknesses will prove to be any nuisance. There's no immediate threats and many pieces have yet to be developed. Game could still go either way. Yes, people prefer Bishops over knights but maybe this dude had other ideas or just preferred knights.  🤔 Only way to discover his plan is to play on.

Black is creating a closed position with staircase pawn structures. Those are better for the knights to maneuver around. So not only does black obtain better pawns; he or she also gains the knight pair. This is a common idea and oftentimes a good move. Note further more that Bxc3+ is a forward move, which does not waste a tempo undeveloping pieces. So you get a dynamic game going with lots of opportunities.

It's roughly an equal trade, although the bishop will be slightly better in some positions.  Your opponent is willing to give up that slight advantage in return for "damaging" your pawn structure. 

What you get in return is more central control and the half-open file for your rook.