This is in reference to the incubator.  The last three days of the 21 days it takes to hatch a chicken egg is lock down time.  The eggs will no longer be turned and it is best to not open the incubator.  NOTE:  I do open the incubator to put more eggs in (using a two incubator system) and also if the humidity makes it necessary for me to add water.  I just do it quickly.