Also commonly known as boric acid or boracic acid, it is a white powder that is used to protect metal surfaces from oxidation while soldering.
The boracic powder is mixed with methylated spirits (denatured alcohol) in a lidded container so that the spirits do not evaporate over time. The powder will settle at the bottom of the container and has to be stirred up before using it.
Jewellery items are dipped into it, or it can be painted onto the item with a brush, and the methylated spirits are then set alight. When the spirits has burned away it will leave a thin protective film of boracic powder over the surface of the jewellery item (also see Protecting gems while soldering).
This is generally only done when working in metals that are particularly prone to oxidation, like silver and 9ct yellow gold, and when there are surfaces that one will not be able to reach to clean up after heating.
Boracic powder can be bought at most pharmacies.