Shelves are flat slabs of fireclay that have been fired to a higher temperature than will be encountered in your kiln. With multiple shelves, you can stack more ware in your kiln than you could ever place on the bottom of the kiln alone.
Half shelves increase kiln space by making it possible to fire tall pieces on one side of the kiln and one or more layers of small items on the other side.
Shelves are strong and will not sag if properly supported and not overfired. But like any other pottery, they will break if dropped.