CMB polarization is caused by Thomson scattering and subsequent gravitational lensing of CMB photons. CMB electromagnetic radiation (EMR) is mostly unpolarized, but somewhat under 10% is polarized. Polarization falls under the polarization modes, E-mode and B-mode (so-named in analogy to the divergence and curl of electric and magnetic fields as expressed in Maxwell's equations: not connected to the Maxwell equations or electricity and magnetism other than by analogy), with unpolarized EMR referred to as T-mode (T for temperature, as the EMR indicates black body spectrum associated with a specific temperature).
Angular power spectrums can be drawn cross-correlating these, known as BB, EE, TT, TE, TB, and EB. The latter two are always zero. TT is just the normal power spectrum, ignoring polarization, and is much stronger as only a small fraction of the EMR is polarized. EE results from Thomson scattering, and BB results from gravitational lensing of the EE polarized EMR. The TE results confirm the existence of the E-mode and have provided a useful tool for study in some cases where the BB results are insufficiently prominent to be of use.
B-modes are also a product of gravitational waves and a prime target of CMB analysis: the identification of primordial B-modes, the presumed product of primordial gravitational waves from quantum fluctuations of the early universe. If found, they would be considered a confirmation of both gravitational waves and inflation cosmology theories.