What are crossed polaroids?

A Polaroid is a material that aligns ordinary unpolarized light into a single plane. Crossed polaroids are simply two polaroids set up so that the second Polaroid is aligning the light into a plane perpendicular to the plane of the first. To put it more simply, the first Polaroid crams the light into the Y axis and the second crams the light along the Y axis into the X axis which essentially reduces it to nothing.

hyperphysics.phy-astr.gsu.edu/hbase/phyopt/polcross.html