Gesso is a primer used in painting and mixed media art. It's a white or off-white paint that can be applied to a variety of surfaces, including wood, canvas and paper.
The best way to use gesso is to brush it on the surface you're working on before you start your art project. One coat is usually enough but if you want extra protection, you can apply more coats. The more coats you apply, the thicker your gesso layer will be which may affect how well your other art supplies stick to it.
After applying gesso to your surface, let it dry completely before painting over it with oil or watercolours or applying other materials such as collage elements or pastels.