Gesso is a must-have for artists. It's similar to white acrylic paint, only thinner. It dries hard, making the surface stiffer. Gesso primes the surface for painting, making the surface slightly textured and ready to accept acrylic paint.
Without gesso, the paint would soak into the weave of the canvas. The beauty of gesso is that you can apply it to nearly any surface, and then you can paint on that surface with acrylic paint. For example, you can apply a layer or two of gesso to vinyl records, rubber objects, wooden surfaces, and voila - you can now paint on that object with acrylics! The possibilities are endless!