Lignin

What is it?
Lignin is a complex chemical substance found in the cells of wood.

The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood.

Certain bacteria and fungi can eat Lignin so helping to decompose the wood so that other higher lifeforms like infusoria in your tank can consume it and the bacteria/fungi and therefore provides food for fish fry, shrimps and catfish.

Links

 * Wikipedia on Lignin