The phrase "large slab" refers to a big and flat piece or chunk of something. It suggests that the object being described is thick and substantial in size. Full definition
Mirrors were made of large slabs of glass with one face ground into a curved shape and coated with a thin layer of aluminium. (kavliprize.org)
Plenty of evidence supports the idea that the Earth's outer layer is divided into large slabs that gradually move over the mantle. (sciencenews.org)
These «slab» paintings led Whitten to his next artistic development: a new kind of tesserae (a material used in the construction of mosaics) made by cutting cubes of color from large slabs of acrylic paint. (art21.org)