Why is this an inaccuracy ?

hello I am 900 rated chess player, I’ve been analysing my games recently to improve my rating but in my last game I was shocked by one my inaccuracies, here I played the move b5 to gain space and stop the bishop from going to c4 but that was actually a very bad move. Can someone explain the idea behind it so I can keep the idea in mind for next time ?

Ok let me teach you how to calculate

Now what was the pawn doing on b7 protecting the knight ofcourse Now what if the opponent plays rook to c8 attacking the knight if we simply ignore then opponent will apply pressure and what ever we do the pawn on c7 or pawn on b5 or a6 will be taken but ignore this because your opponent didn't see this probably. Also the b or c file will be opened and open position means better position for you only. Since you have more pawn there but White's whole attack is on kingside so that's a bad file, maybe you can make passed pawns And finaly the pawn on c7 becomes isolated pawn and is pinned by the rook so it will take 2 move to protect it and make it normal also pawn to b5 creates to c Backwards pawn on c and a file

What to do when calculating. B5 will create an isolated pawn B5 also cause two back ward pawn supporting pawns Also the rook will cause some troubles Think what your opponent will play ex after. Ne7 opponent will have no special strategy but b5 gives strategy to put pressure and bully your isolated and alone pawn 😭.

ShinnyPanda has given you a good explanation there. It is an example of Don't only think about what your move will do, but also what will no longer be done. So look at what a piece is doing where it is already before moving it. If the costs outweigh the benefits, find a better move.

It's weird that it was not a mistake. This move is absolutely unnecessary, weakens your pawn structure, gives you nothing. If your opponent wants, he could even at some point play a4, applying pressure to your pawn. Of course, not just yet, as you could play b4, and at some point create a protected passed pawn. But you know, it weakens your pawns really much. I would probably in this position play Rfe8, I believe it's a normal move, although not the best according to the engine

The explanations above are great. I also think bishop c4 was never a threat because white simply loses a pawn and their king safety if the bishop move.