Chainmaille is the art of weaving together many, many (many, many!) small rings into a larger whole. Formerly used as armor, it is now used in jewelry and costuming.
Chainmaille has multiple different weaves. Some of the most classic weaves, including European 4-in-1, have been around for a very long time. Others are relatively newer. But they all involve carefully opening each ring with pliers, weaving it into place, and closing it up carefully, to disguise the seam as much as humanly possible.
Chainmaille can take the form of a single chain, or a larger flat sheet, or even 3D pieces.