From The Aquarium Wiki
What is it
Sumatra driftwood is dry wood supplied to retail shops. Unlike traditional wood offered in aquarium shops, this item has multiple points of branches radiating out from a centre point.
It is collected from the wild, dried and sand blasted for shape and size. It is believed to be collected in an ethical manner. Like commercial bogwood this item will float for a short time. While it will release harmless brown colouring called tannins into the water, this soon passes after several weeks.
What is it for
Like bogwood, driftwood is used for an aesthetically pleasing effect in the aquarium. The wood becomes a focal point of interest and provides a safe haven for aquatic life in the aquarium leading to healthier animals.
The tannins released are actually beneficial to aquarium inhabitants as they supply trace elements missing from the fish's normal environment. The tannins suppress bad bacteria and fungus and the wood has been shown to help feed fry, some species of pleco and invertebrates.