Understanding this move

I think that it's because if the pawn takes on e5, Ne5 and puts more pressure on the queen?

The difference according to the computer is only .3 difference which is not much at all. For your rating the difference in how much better the move is does not matter. Also Bg4 is a good move as it develops with tempo. E5 does challenge the center, and forces white to make a decision on how to deal with the threat. It also a developing move as it allows you to develop your other bishop, and does not block the light squared bishop. E5 is just as much of a developmental move as Bg4 is. You still have to make at least one more pawn move to be able to fully develop your pieces. I believe the reason why the computer says that e5 is slightly better is not just because it is just as much of a developing move as Bg4 is, but also you are further in development and white is at least 3 moves away from castling. So e5 is a way to open up the center a little bit, and that can give you an advantage. This can also lead to tactics that work in your favor. e5 is a way that accomplishes just as much as Bg4, but weakens white's king safety a little bit for the time being. That is an advantage for you. In a game though I probably would have played Bg4 as I would have developed with tempo, and would want to castle myself before trying to open up the center. Hope this helps.