מסך, Masach , Curtain
‘Masach’ means ‘Curtain’, ‘screen’ in Hebrew. In simple words, It is a spiritual ‘border’ that is created when we raise a desire to draw light from the higher levels to the lower level, Malchut. The Masach push away the light, holding and restricting it from entering directly into the vessel because direct Light could break the vessel. The ‘Masach’ also creates separation between worlds and levels.
The Zohar (Beresheet) gives an example of physical ‘Masach’ in the body, which is the membrane between the lungs, the heart that sustains life, and the lower organs that ‘manage’ the processing of physical nourishments.
There is also the aspect of ‘Mashach’ in our brain, separating consciousness and unconsciousness. The unconsciousness is the force of life that control the billions of operations in the body. If this ‘light’ was open the consciousness we would break in an instant. Connection to the unconsciousness can be done under a sleep state, when the ‘consciousness’ is ‘dormant’, meaning out of the way, and even in that state, we have only a small ‘windows’.