He was captured by rebel forces and kept as their 'prisoner' for several months before they set him free.
They 'took him prisoner'. = He was 'taken prisoner'. [=they captured him and made him their prisoner]
a 'political prisoner' [=a person put in prison because of his or her political beliefs]
(..) to do with this? I just... I wanna help. All yours. They expect us to just sit here like prisoners. We should be out there looking for him. Honey, we have to trust them, okay? This is our government. They're on our side. That man gives me the creeps. (..)2016 TvShow: Stranger Things Title: Chapter Seven: The Bathtub Season: 1 Episode 7