"Cardboard sheets" refer to flat pieces of thick paper material made from recycled pulp, often used for packaging or crafting purposes. Full definition
Analysts believe Nintendo «s plan to launch «Labo,» LEGO - style accessories for the console that kids can build themselves on cardboard sheets, could bring in a younger audience. (fortune.com)
On a high level, Labo works by providing a set of pre-cut cardboard sheets that you punch out, fold, and connect in order to build different attachments called Toy - Cons (ADORABLE name btw). (engagedfamilygaming.com)