Exopolitics is a fancy term for "extraterrestrial politics" (the Greek prefix "exo-" means "outside" or "external - compare exoplanet). As no extraterrestrial life has been discovered yet, let alone extraterrestrial life capable of politics, this is the domain of cranks and
nutjobs some really weird people. It's the result of what happens when beliefs progress beyond the simple fascination with UFOs ("there's no other explanation for UFOs, therefore they must be alien spacecraft") well into true believer territory ("of course we know that there are multiple alien races!").
The term has been used to denote two slightly different things - the alleged dealings of aliens with Earth politicians (e.g. Michael Salla's claim that Eisenhower met aliens) and the science fiction-like tales of alien wars and alliances. If this sounds vaguely familiar, both meanings form a lot of the plot of The X-Files...