What is the meaning of polygamous cult?

Polygamy means that you have more than one spouse, and usually means a man has more than one wife. Cult can be defined as "a particular system of religious worship bound together by veneration of the same thing, person, etc." or "a religion considered to be false, unorthodox, or extremist, with members living outside of conventional society usually under the direction of a charismatic leader". So, generally speaking, a polygamous cult is a group of people that many would consider unorthodox and live outside conventional society that accepts and practices the idea of having more than one wife.