Saggers are reusable fired clay boxes into which pieces of glazed pottery are placed during the firing process. They protect the glazed ware from the atmosphere of the kiln. They're usually used in kilns which are fired with "dirty" fuels such as wood or dung. Saggers prevent the ash and other impurities which circulate in such kilns from settling on the pots and marring the glaze. Saggers are not necessary when firing in electric or gas kilns.
Saggers are also used by some art potters as containment vessels to trap color-producing fumes given off by various materials such as metal shavings and colored berries. This is a relatively low-temperature technique so it's not necessary to use specially made saggers. Terracotta flower pots with shards of clay covering their drainage holes are adequate.