A Plasma window is a field of plasma that fills an area of space. It is similar to a force field. The area is created by using magnetism to hold the plasma in place. With current technology, this area of space is quite small. It is in the shape of a flat planes inside a cylinder.
As plasma increases in temperature, it becomes thicker. At a high enough temperature, plasma is thick enough to separate a vacuum from normal atmosphere. Plasma does not stop radiation, such as lasers, from moving through it. Because of this, scientists can use a plasma window to create an area of vacuum while still letting radiation to move through it. This is needed because certain types of radiation can only be created in a vacuum, but scientists need to use the radiation on things that are found in a normal atmosphere.