To Prime a surface ensures a better base for your paints to adhere to the canvas. Acrylic gesso is a combination of calcium carbonate ,chalk, with an acrylic polymer medium, a pigment and other chemicals that ensure flexibility, and ensure long archival life. “Pre-primed” means “pre-gesso” It already has a gesso layer applied in the factory. On some of the pre primed ranges manufacturers often use a seal on top which creates a shiny surface a coat of gesso will remove this making an ideal surface to work on. If you would like a super smooth surface then you can sand between the coats.