Gesso is what is commonly used to prime your canvas. By prime, I mean getting it ready to paint on. The purpose of gesso is to stop paint from sinking into the canvas weave (which would quickly deteriorate the canvas) and provide a responsive surface to paint on.
Most canvas you buy from the art store is already primed and nothing further is needed from you before you paint on it. However some artists prefer to add some additional layers of gesso.
Gesso is similar to a white acrylic paint, but it is generally thinner and dries harder.
The texture of different brands of gesso varies, with some being more smooth and refined and others more rough. You can add additional rough texture and variance by the way you apply the gesso.
Gesso is generally applied in a number of layers to build up a thick base on the canvas. A large brush and / or palette knifes can be used to apply the gesso.
If you are painting a landscape then a more rough gesso background may be preferred to help render the harshness of the environment. For more delicate subjects such as portraits, you may prefer a more smooth and refined gesso background to work on.