To do something unnecessary; to do something that is redundant or duplicative. (Newcastle is an English town from which coal is shipped to other parts of England.)
In other words "to take something to a place where its kind exists in great quantity."
1. Taking food to a farmer is like carrying coals to Newcastle.
2. Mr. Smith is very rich he doesn't need any more money. To give him money is like carrying coals to Newcastle.
3. Exporting pine to Scandinavia is a bit like carrying coals to Newcastle.