To frog-march someone is to force them along in a quick march, usually with two escorts, one to either side, each holding one of the person's arms behind their back. Naturally, this is most often done by police escorting a prisoner, keeping them firmly under control.
The origin of the term comes from the rather more enthusiastic police of Georgian era England, who would 'escort' drunken or dangerous prisoners four-to-a-man, each taking one limb and carrying the prisoner spread-eagled between them. Apparently the splayed helplessness of the position reminded onlookers of a frog, and the name stuck.