In the strictest sense, the Italian word gesso refers to a mixture of calcium sulphate (a white powder commonly known as gypsum) and rabbit-skin glue that is applied in liquid form as a primer to the surface of a painting support. In broader usage, the term has come to refer to many types of similar white preparatory layers, including those made from chalk (calcium carbonate), as well as modern synthetic adhesives.
The gesso layer smoothes out the irregularities in the support; it is usually applied in many thin layers and sanded, scraped, and polished between applications. The final smooth surface provides an optimum base for painting in a variety of media.
Source: Looking at Paintings: A Guide to Technical Terms / eds. Tiarna Doherty and Anne T Woollett. (Los Angeles: J. Paul Getty Museum, 2009), p. 36.