Merchant account is something that allows you to accept, refund, and transfer payment. Many top credit card processing companies in United States requires you to get a merchant account before accepting credit card.
To qualify for a merchant account, it is needed to open a U.S. business checking account. That means, you will have to set up a company with a valid address in the country. Although it is not a bad idea to register a U.S. company, for most small business, it costs long time and maybe a big money to do so.