It is a stainless steel pipe, either rigid or flexible, made for relining flues of masonry chimneys when the original clay liner has cracked or broken. It may also be used to create a lining in a masonry chimney that was made without a clay liner.
Liners are essential because they channel heat and gases up through the chimney top and block the heat from causing combustible materials around the chimney from catching fire.
Most people think of chimney liners only in terms of fire safety: Especially if you burn wood, you probably know that an unlined chimney, or a chimney with a damaged liner, is a fire hazard. Chimney fires burn hot enough that combustible forming around the chimney can catch fire right through a layer of brick, therefore, a liner provides extra protection to the interior of the chimney and helps guide the gases and particles up and out of the structure.