Meaning - To do something that is obviously superfluous
The phrase, "carrying coals to Newcastle," means spending an inordinate amount of energy on something useless, fruitless, or redundant. This idiom arose in the 15th century because Newcastle, England was known throughout the country as a major exporter of coal. Therefore, "carrying coals to Newcastle" would do you no good, because there was more coal there than anywhere else. Variations on the saying include "bringing," "taking," or "moving" the coal.
Other countries have similar phrases; in German it's 'taking owls to Athens' (the inhabitants of Athens already having sufficient wisdom). 'Selling snow to Eskimos', which in many people's understanding is also the same, has a different connotation. Both the Dutch and Spanish having sayings, 'like bringing water to the ocean'. In Poland and Sweden, you'd hear, 'bringing wood to the forest'. Some regionally specific idioms for redundancy include Russia's 'taking samovars to Tulu,' a city famous for its spigotted teapots.
Ironically, in 2004 Newcastle began importing coal from Russia.
Sources: Wisegeek.com, Phrases.org