The pages of type were arranged and clamped in a frame, so that when printed on a sheet of paper the full size of the press, the pages would be right side up and in order when the sheet was folded, and the folded edges trimmed.
This passage describes a method used in printing where pages were carefully arranged and clamped within a frame, ensuring the correct positioning and order of the pages when printed on a sheet of paper. The folded edges would then be trimmed after the sheet was folded.