SAR is an active collection system that emits a radar signal and measures what bounces back. Different combinations of vertical and horizontal wave polarities are used to measure and detect different kinds of phenomenon and features. Capella Sentinel-1 based Change Monitoring uses two different polarities available from Sentinel-1: VV (vertical-vertical) and VH (vertical-horizontal).
VV is a mode that transmits vertical waves and receives vertical waves to create the SAR image.
VH is a mode that transmits vertical waves and receives horizontal waves to create the SAR image.
The following shows an example of vertical and horizontal wave patterns.
Different polarity combinations are more and less sensitive to different kinds landcover features and phenomena. For example VV modes tend to pick up height/vertical features where VH modes tend to be more sensitive to surface textures.