A pellet stove is a stove

Stove

A stove is an enclosed heated space. The term is commonly taken to mean an enclosed space in which fuel is burned to provide heating, either to heat the space in which the stove is situated or to heat the stove itself, and items placed on it...

 that burns compressed wood or biomass pellets

Wood pellets

Wood pellets are a type of wood fuel, generally made from compacted sawdust or other wastes from sawmilling and other wood products manufacture, but also sometimes from sources such as whole-tree removal or tree tops and branches leftover after logging and which otherwise help replenish soil...

 to create a source of heat for residential and sometimes industrial spaces. By slowly feeding fuel from a storage container (hopper) into a burn-pot area, they create a constant flame that requires little to no physical adjustments.
Scrap wood and ship-lap burners have been around for decades. Barrel stoves, braziers, and oil drum fires in Depression-era Hoovervilles
support this.