A cleanroom is an enclosure that controls the amount of particulate (dust and other contaminants) in the space.
It does this by controlling a number of factors;
The build materials and surfaces in the room need to be designed for easy cleaning and to limit the areas that particulate can build up.
Controlling the air entering the space via a filtration system. The final filter will be some sort of HEPA filter. The cleaner the space the more often you will want the air to pass through this filtration system (increasing the number of air changes per hour)