Rabbit skin glue is made of rabbit collagen, present in animal proteins, which helps to hold the rabbit together. Collagen isn't just found in rabbit skin, but also in tendons, bones and teeth. Rabbit protein can have about 25% to 35% collagen. When placed under a microscope, collagens look like rope. When not holding a rabbit together, rabbit collagen can act to hold paint to a surface when mixed with a gesso. It also tends to give a glaze to the finished painting.
Rabbit skin glue has been used by artists for centuries to help keep paint on a surface, such as fabric, clay, wood or canvass. It was traditionally made from the hides of rabbits, as the meat and bones was used for food, but sometimes the whole carcass was used. It is unknown when rabbit skin glue was first used, but the first reference to glue making from animal parts was from 200 BC. Paintings from the renaissance commonly used rabbit skin glue and linseed oil paints. Linseed oil can destroy a canvass over time, but the rabbit skin glue was found to protect the canvass, even centuries later.
Rabbit skin glue is used in other ways other than for paintings. It's also used in woodworking, musical instrument making, glue and sizing needed for repairing artwork or furniture. Sizing is means to keep paint, varnish or stain on the surface without damaging the surface material.
When you paint a prepared canvas, it is wet and flexible, but when it dries, the shrinking will distort and ruin the painting. This is why you need a gesso and something like rabbit skin glue as your primer. It can continue to absorb any humidity in the air around the painting to keep the canvass moist and strectched out. However, over centuries, paintings will eventually crack and the paint will start to flake or chip off.
Rabbit skin glue should only be used for oil paints and not acrylic paints. Over time, acrylic paint flakes and falls off of any canvass where rabbit skin glue is the primer.
Rabbit skin glue is also more difficult to prepare than other primers. It needs to be mixed with water, heated (but not boiled) and needs to be worked with while warm.