Opus tessellatum refers to a type of mosaic
Mosaic is the art of creating images with an assemblage of small pieces of colored glass, stone, or other materials. It may be a technique of decorative art, an aspect of interior decoration, or of cultural and spiritual significance as in a cathedral...
made from tesserae. Opus tessellatum is usually used for background consisting of horizontally or vertically arranged lines (but not both; that would be "opus regulatum").