Ever wondered how circular polarising filters work? Probably not but the section "Polarized Light" near the top of this page
has a fairly straightforward explanation.
I'll risk going even simpler by summarising and saying that it is a linear polariser (similar to those used with some sunglasses to cut out glare
) immediately in front of a gizmo which converts the linearly polarised light into circularly polarised light. The point of doing that conversion is that sensors such as are used in a camera's autofocus system need unpolarised light to work properly and they can't tell the difference between unpolarised and circularly polarised light.