ESTA stands for Electronic System for Travel Authorization. The ESTA system operates under the Visa Waiver Program, which includes 39 countries, whose nationals are eligible to apply for ESTA travel authorization to travel to the United States.
This requirement is only valid for non-U.S. citizens. American nationals are not required any additional travel document but their American passport to enter the United States. Therefore, if you are an American citizen, you do not need an ESTA to travel to the US.