Complex sentence: A master layout "card" known as the "background" was shown behind the transparent areas of each card, which could be shared as a layout among several cards, but with card-specific content.
Split into simpler sentences: A master layout "card" known as the "background" was shown behind the transparent areas of each card. Objects placed on it, like fields and buttons, would be copied o, which could be shared as a layout among several cards, but with card-specific content.