From AMS Glossary
The vegetative matter, such as leaves, twigs, dead logs, etc., that covers the ground in the forest; unconsolidated decomposing and partially decomposed organic material immediately under a layer of leaf litter.
Duff forms a layer about 5 cm (2 in.) thick that overlies the soil of a forest floor. Its thermal insulation is an important factor in the formation of permafrost, and the quality and moisture content of duff is significant in considerations of forest fire hazard (fire weather).