The forme (form in US) is all the hot metal type, blocks (cuts), and furniture to be printed in one impression locked up after imposition in a chase ready to be moved onto the bed of the press.

This example is not too huge and is going to be printed 4 pages to view. Most book work was done 8 or 16 pages to view which made the formes immensely heavy and unwieldy. They would be wheeled from composing room to press room on metal trolleys constructed at the same height as the bed of the press, so that the forme could be slid into position upon arrival. (It looks like this one is being balanced on such trolley.)

In the early days of printing, when type was often scarce a compositor might set in forme order. In this picture, you are looking at, say, pages 7, 2, 6, and 3. So the comp might deal with those pages first, and then set pages 1, 8, 4, and 5 for the back-up of this sheet. After the first forme was printed the type could be distributed and be available for the comp’s next batch of pages. This procedure would call for accurate casting-off (or compromises on page length consistency).