What is gesso
Gesso is a white paint mixture consisting of a binder that is mixed with a combination of chalk, gypsum and/or pigment.
Gesso acts as a primer for paintings on wood panels, canvas and sculptures.
As an acrylic painter I use the modern "acrylic gesso", usually the Liquitex® White Gesso. However this is not technically a gesso because it is a combination of calcium carbonate, acrylic polymer, latex, pigment and other chemicals. These help ensure flexibility or archival life. It is non-absorbent, thus creating a perfect layer on which to place acrylics.
When using gesso it is best to apply thin coats (it generally does not dry as fast as a layer of acrylic paint would) in one direction. Keeping it smooth and flat is bested, unless you are going for a more textured painting. Let it dry completely before applying another coat, or adding acrylic paint over it.
You can also purchase canvas that has already been primed with gesso.