"Opposite polarity" refers to when two things are completely different from each other or have completely different characteristics. It means they are opposite in nature or direction. Full definition
According to the leading theory, a flare is set off when nearby magnetic fields of opposite polarity break and reconnect, creating a closed loop with two footprints on the solar surface. (discovermagazine.com)
Two structurally similar halves span the membrane with opposite polarity. (science.sciencemag.org)
The ST - IAT is a version of IAT (Greenwald et al., 1998) that measures the level of association between two categories characterized by opposite polarities (e.g., pleasant / unpleasant) and a single target category (in the present study the category was «mother»). (frontiersin.org)