Why is d5 a good move here?

Your Welcome!

Also to bei fair it's way more easy to try to explain a move than playing it

d5 makes sure you gain space advantage, while minimalizing the pressure your opponent had on your center (Nc6, Bg4, pawns c5 and e5). After d5 Ne7 Bb5+ Bd7 Bxd7 Qxd7 you enter a position where you have a (theoretically) good bishop (pawns on light squares, dark squared bishop) compared to black's bad bishop (dark squared bishop), having space advantage and no black pieces that could create problems for you. With a closed center, you would aim to play on the flank. Additionally, your pawn on c3 would not allow his knights to potentially land on d4 square where it would be best to exchange it.

Conclusion: with d5 you gain space advantage and closed center with good bishop against bad bishop. Strategically, this is quite an advantage which you may use for favorable middlegame and endgame.

Thanks! I understand now, you all have helped a lot, thank you so much :D

@Dario19503 I agree with your points I think tho that you should play c4 immideatly after you take d7 because how can he get his knight to d4? I think it's not possible without whits help. Also black might play b5 and c4 and then then maybe a knight could occur on d3 at some point from c5 to d3. Also in general its a good idea to put the pawn on the opposite colour of your bishop if you only have one because it gets more power. So maybe after b5 and c4 he could maybe also activate his bishop from d8 to b6 where the bishop would stay ok.

You should not push pawns if it's not needed, it takes some time and you could do something else instead. I took a quick look, explaining the idea about the suggested move, but did not give analytical insight any further as I didn't feel like it was needed.