Yo what's up, I'm a beginner too (950 elo).
To me it looks like the move d5 allows white to respond with e4, attacking the d5 pawn and opening white's light-square bishop for development. It's an issue because, let's say you play a natural move such as Nf6 to defend the d5 pawn, the bishop would then go to b5, forming a battery with the queen and strengthening your knight's pin to the king.
Those are my thoughts, hope this helps you
I believe the main reason why e5 is recommended is because it is a tempo move (attacking the bishop forcing them to move it), and is a developmental move for you too. Would allow you to develop the other bishop, and queen. This would lead to a lead in development, and allow for quicker castling (king safety is king pun intended) which is advantageous for you. Moves like e5, Nf6, either Be7 or Bc5, and then O-O would probably what follows for you if you played e5.
Ballistic7844's reason are also valid, but I don't think that is the main reason due to the being defended enough.
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Sorry if this is rather easy im new and learning,I went on to relieve the pin with bishop to d2 and that was the best move AFTER this mistake. In my head I needed to deal with that pin right away and try to kick the queen around