Gesso is a primer. Artists sometimes apply it to a surface before painting on it. In fact, gesso looks a lot like paint. Originally, gesso only came in white. Artists put it on canvas, wood, or other surfaces before creating a painting with oil paint or acrylics. Gesso makes the surface a little stiffer. It prevents paint from soaking into the support (canvas, paper, wood, etc.), and it gives the surface a little more texture (called "tooth"), so the paint sticks better. Please see related link.