Why does white give his knight for pawn?

I played a game with a good player outside chess.com , The game went like this 

There is no reason to play Nxe5 here.   Black should capture the knight.  

After 6...Nxe5 7.d4 Black can play Nxc4, keeping his extra piece after 8.dxc5.

If he plays that move, you should stop considering him a good player.

No, after some moves he re-took my material but with a pawn up.

Show us the moves.   We'll show you the defense.   

this is the fork trick

Yes, it is the fork trick, but mis-played. The bishop on c4 is hanging if it is tried.

Edit: Actually, it's called the Knight Trick if i'm not mistaken.

There are some positions in which Nxe5 wins a pawn using a fork theme. But this isn't one of those.

This would normally be called a center fork trick but white's move is a blunder in this position as you can play nxe5 and when he plays d4 forking your bishiop on c5 and your knight on e5 you can play nxc4 and be a peice up

Everyone's idea of a good player is someone who beats them.

And you made the "really" up yourself.

See white won the pawn. How to counterplay this trick?

You should've taken the bishop on move 7.

Both of you played awfully in this game. Read some good books and get some practice. You'll eventually improve you're so young.

Oh! I missed a 6 point advantage and fell short of a pawn

black should capture the knight in position 1