Chunks to burn in a boiler/stove. They can use everything from sticks up to small "logs" (under 3n inches I imagine). It seems to be more popular in eastern Europe, at least all the videos of that style of machine seem to be out of the east.
Heres is the style of chunker that appears common in the west of Europe (Scandinavia) . Notice it can take a larger log. Id love to build this style if I had a tractor with enough HP/ So fast!