Radar is a common tool in remote sensing, it is a so-called active instrument, hence it emits and receives energy. Modern Syntethic Aperture Radar (SAR) systems are capable of both transmitting and receiving polarized waves. The most common polarizations are horizontal $(H)$ and vertical $(V)$ polarization, although other ones exist. Using H and V leads to 4 possible combinations of emitting and receiving signals:
$$HH, VV, HV, VH,$$
which is read as transmitted first, received later (i.e. $HV$ means transmitting horizontally polarized waves and receiving only vertical polarized waves).
I can relatively easily imagine the use of the $HH$ and $VV$ copolarized waves. They should be easier reflected by object that have a longish geometric structure. Large $VV$ and small $HH$ would probably indicate tower-like structures and small $VV$ and large $HH$ should indicate bridge-like structures.
However it is not obvious to me what structure would reflect a cross-polarized wave $HV$ or $VH$ stronger than $HH$ or $VV$? Furthermore, what is the difference between an object that reflects $VH$ and an object that reflects $HV$?