Can anyone explain this magic trick?

In order to "see" something, light has to hit it first and bounce toward
your eye.
When the glass is empty, some light passes through the glass and
hits the coin, and it bounces back to your eye. You "see" the coin.
Simple and straight forward.

But this magic trick takes advantage of an important property of light:
when light hits a boundary between two materials it bends a little
bit. In this case, filling the glass with water creates a boundary between
two materials: the air and the water. So now we have the following
events happen to the light as we follow its path:

1) light passes through the water and exits the bottom of the glass
where it bends.
2) some of the light hits the coin and bounces back up toward the
water where it bends again.
3) the light exits the water and passes into the air through the side
of the glass where it bends again.

The net effect of all this bending is that there is no longer any light that bounced off
the coin still traveling in the direction of your eye because it has
been bent too much. So the coin ‘disappears’.
But all you have to do is change your position to where the light off
the coin is now bending and you will be able to see the coin.

It is no different than trying to catch a fish in the water using your
hands. You don’t aim
for where you "see" the fish, because the water/air boundary has bent
the light. You compensate for the bending of the image and aim for where the fish would be if the light were not bent.