Light exhibits polarization, its electric field of light can vibrate in many different directions relative to the direction of the light's travel. This picture depicts the electric field of a radially polarized wave. The circular nature is a result of the radial symmetry of this type of light.
The electric field of light is complex, so it has both real and imaginary parts. Since the electric field is a vector, it can be described with x, y, and z components. This picture is specifically of the real parts of the x and y components of the electric field, overlaid on top of each other.