Starlight reaches us after having traveled through the magnetized, dusty Inter-stellar Medium (ISM). On its journey, starlight interacts with the interstellar dust. Part of the light becomes absorbed by the dust grains. Because the grains are asymmetric and elongated, they preferentially absorb light along their long axis. This causes the light to become polarized along the short axis of the grain. As dust grains are aligned with their short axis along the direction of the local magnetic field, the starlight's polarization is also parallel to the magnetic field.
Polarimetry, as the name suggests, is the science of measuring the light's polarization. The most significant quantitative measure of polarization is the degree of polarization (DOP or p). DOP, in simple words, measures the portion of light that is polarized. Based on that quantity, light can be polarized (p =100%), partially polarized (0% ≤ p ≤ 100%) or unpolarized (p = 0%).