Here segnature refers to the imposition process, i.e printing different pages on a single press sheet in the order required so that after folding and cutting the pages end up in the correct reading order.

In a book for instance there would be 1 signature for the 4 pages used for the cover (the "4 [pagine] di copertina") in your text) which will need to be printed in color or on different stock, and the signatures required for the inside pages.
The number of pages per signature depends on the equipment and folding method used (as does their order and orientation), and for a single publication there might be different types of signatures depending for instance on the use of different paper for certain sections or of the use of color for certain pages.