The 1955 doubled die cent is a die variety that occurred during production of the one cent coin at the United States Mint

United States Mint

The United States Mint primarily produces circulating coinage for the United States to conduct its trade and commerce. The Mint was created by Congress with the Coinage Act of 1792, and placed within the Department of State...

 in 1955. This variety is often mistakenly referred to as an error. Error coins occur on a per strike basis, while die variety coins are produced for every strike with a specific die pair as is the case with the 1955 doubled die cent.
When a modern coin die is created, it is struck from a working hub, which places the incuse image onto the die that will subsequently be used to strike coins.