A U.S. visa is a document that permits you to attempt to arrive at a port of entry to the United States. Having a visa does not guarantee entry to the United States. The final decision comes down to the CBP officer reviewing your case.
The CBP officer will interview you upon arrival at a U.S. port of entry. Your documents and luggage may be checked. If the CBP officer suspects you of being untruthful, or if you have lied on any part of your visa application, then you may be refused entry to the United States even with a visa.