According to the Oxford Dictionaries, a "white elephant" is a possession that is expensive and hard to maintain but also difficult to get rid of. For example, it may be used in reference to a house or car that requires a lot of upkeep and cannot be sold because no potential buyers can be found.
The origin of the phrase comes from a story about the kings of Siam, who would give live white elephants to members of the court they disliked in order to ruin them with the cost of keeping the animal. In actuality, white elephants were a sign of power and prestige in the counties of Southeast Asia. They were owned by monarchs who ruled justly and maintained a peaceful and prosperous kingdom.
In the United States, white elephant gift exchanges are a popular holiday practice. According to Better Homes and Gardens magazine, the participants in the exchange brings an item from home that they no longer use or need to serve as a gift. The presents are exchanged between all participants, who can then swap gifts in an attempt to get exactly the present they want. The game hinges on participants bringing unusual items that other players may find valuable or desirable.