It's a Latin term, meaning "in chambers" or more generally, "in private." A judge's private office is known as his or her "chambers," as every viewer of Law & Order knows from the recurring scene in which a fed-up judge barks to the lawyers "In chambers -- now!" It's the legal equivalent of being taken to the woodshed. When the parties disagree as to the admissibility of document, the judge will often conduct an "in camera inspection" of the document, far away from the jury's gaze.
By the way, the judge's camera is related to Kodak cameras. The photographic variety is a shortened version of camera obscura -- a darkened room into which light is admitted through a double convex lens, casting an image on a suitable surface.