"A circular polarizer consists of a linear polarizer followed by a
quarter-wave plate. The linear polarizer selects exactly the same
components of the incoming light that any linear polarizer does (ie, the
effect on the scene you are photographing is the same). The quarter-wave
plate then splits the linearly polarized light into two components (each
at 45 degrees to the axis of polarization), and retards one component by
a quarter wavelength with respect to the other. This has the effect of
turning the linearly polarized light into circularly polarized light. So
the selection criterion for the filter is the linear polarization state,
not the circular one."
This is the correct explanation. (And a clear one, I might add.)
The same principle is used in radio antennas, particularly for FM
stations. If a station's antenna is horizontal, the vertical antennas on
automobiles don't do a very good job of picking up the signal (and
drive-time ad revenue is critical for keeping many stations on the air).
So, by using two antennas that are one 1/4-wave out-of-phase *, you
generate a circular wavefront that has both vertical and horizontal
components, and can be picked up regardless of the receiving antenna's
orientation.
In a camera, circular polarization allows light to pass through any
optical component that would discriminate against light polarized in
only one direction.
* As I write this, I suddenly realize that I don't remember the exact
configuration, and my technical books are packed away. So don't jump on
me if this explanation isn't wholly correct.
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