An ice shelf is a floating ice mass that is attached to the coast along at least one edge. Ice shelves form when large areas of land based ice sheets have flowed to the coast and then floated in the ocean. Either the ice will stay attached to the ice sheet as an ice shelf, or it will calve (break off) to form icebergs.
Much of Antarctica is fringed by ice shelves and of these, The Ross Ice Shelf is the largest, covering an area about the same size as Texas or France. Ships in Antarctica try to steer clear of the ice sheet edge as having an iceberg calve onto your Ship can be quite upsetting!