Gesso is a white paint mixture consisting of a binder mixed with chalk, gypsum, pigment, or any combination of these. It is used in artwork as a preparation for any number of substrates such as wood panels, canvas, and sculpture as a base for paint and other materials that are applied over it.
A gesso layer is optional. Many artists choose to work on gessoed grounds so that they are starting on a white surface. Artists working in encaustic often paint directly on raw wood and other absorbent substrates without gesso.
You can either purchase pre-gessoed boards or purchase encaustic gesso and apply it to the ground yourself. As a general guideline, grounds for encaustic painting must be absorbent, so acrylic gessoes are not recommended. Rabbit-skin glue gesso is considered the most traditional, time-tested ground for encaustic.