This is a special room that is assembled inside of the workshop to keep the needed environmental standards (low dust, etc.) – that is why it is called “clean”. It is like the inside of an operation room at a hospital.
We need this room to prevent any impurities from entering the product/production process from outside, so that we can keep the purity until we have packed the quartz.
Here is some more information: https://en.wikipedia.org/wiki/Cleanroom and http://www.cleanairtechnology.com/cleanroom-classifications-class.php