Whats the idea behind this move (b3)

stockfish played the move b3 here, I assume only grandmasters can understand it, my personal opinion is that its a waiting move

thansk a lot

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Hello 😁

It is because you are preventing black to pursue their idea of putting the knight on the C4 square and more importantly this was the only natural way to complete your development by placing the dark square bishop to an active diagonal. It was so powerful that it worth more than 3 points and that's why your opponent decided to trade it for their dark square bishop. If you want some structured help (game reviews, common mistakes, etc.), I work with beginners and under-1200 players. I’m not big on theory — more on fixing practical issues. Feel free to DM me if you want to talk it through.

If you think where you will put your D2 knight, then the correct plan is to first go on F1 and then G3 square

Thats good

It's a typical Ruy Lopez idea mainly directed against Na5 which is bit offside atm.

White wants to play the thematical Ruy Lopez maneuver Nf1 + Ng3 but the immediate 1. Nf1 would allow 1... Nc4 reactivating the knight whereas after 1. b3 it may take long time for the black knight to find better squares.

It also prepares Bb2 but that's only secondary idea and I am not at all sure that is where the bishop will end up. It may prove useful to keep the bishop in the c1-h6 diagonal instead because in some continuations there could maybe even be a chance to sacrifice the bishop to h6.

It prevents the knight from moving further. The only safe move for the knight is to retreat, but the pawns can be traded so yeah IDK

I'd weaken the dark squares a bit more before I attack the exposed king with just bishops or attack with a different developing move TBH, probably use bishops for two weaknesses and then bring more pieces into the attack. Why would you lock the bishop in with the knight anyway?