Waves that are especially simple can be completely described by stating the direction in which they propagate, their speed of propagation, frequency, and amplitude. But there are also simple waves where these quantities are not sufficient for a complete description – for these waves, the oscillation has an orientation in space. This orientation, which is called polarization, needs to be specified as well.
For example, for electromagnetic waves, the polarization describes the directions of the electric and the magnetic fields. For gravitational waves, the polarization describes the orientation of the two orthogonal directions in which distances are maximally stretched and squeezed as the gravitational wave passes.