It's a drainage outlet or floor trap.

Of course you need an outlet for water from the shower to drain off.

If there is no outlet, install the shower screen to sit on a raised kerb (say 50mm) with brick and cement mortar
The kerb keeps the water within the shower cubicle. Create a recess (50mm) through the kerb for the water from inside
the cubicle to drain out to the floor trap.
The tiler has to lay the floor tiles to fall to the direction of the outlet.

This post has been edited by tehtmc: Feb 18 2012, 10:20 PM

The glass partition installer does not provide the kerb. You have to get your tiler or mason to do it.
The kerb as a base is also good to keep the glass panel clean, being above floor level.