Masa paper is a type of machine-made Japanese art paper. It is soft and white, has internal and surface sizing. Masa can be used for a wide range of art techniques, including sumi painting, watercolor, and printmaking. One side is smoother and shinier than the other. Although masa paper is sometimes called rice paper, it's made from sulphite pulp.
A popular technique with watercolor painters is to scrumple up a sheet of masa paper, pop it into a container of clean water for half-a-minute or so, carefully straighten it out again, and then drop some paint onto it while it's still wet. This turns the sheet of paper into one with a textured, wrinkled surface with random color. You can then flatten the sheet out completely using a brayer, and leave it to dry. Mark which is the shiny side before you crumple the paper, so you can tell which side you want to use when you flatten the sheet out. Once it's dried, you may want to glue the crumpled sheet of masa to another piece of paper to give it more support.
