Circularly polarized light is a particular combination of horizontally and
vertically polarized light in which one linear polarization lags behind the
other by a 90-degree phase shift. It's really best to look at a diagram
which you can usually find in a reference or text book on optics.
It is light in which the electric field rotates 360 degrees around the
axis defined by the direction of travel of the light. For linearly
polarized light the electric field is always oriented in one direction
but for circularly polarized light the orientation of the electric field
rotates around the direction of travel.