The Antifederalists were the people who were opposed to the ratification of what is now the Constitution of the United States. They preferred to continue to have a system more like that created by the Articles of Confederation.
Antifederalists were people who feared strong central government. They drew their support mainly from the "common people." They felt that the new constitution would set up a system that would be used by the rich to abuse the common people. They felt that the new central government would be less responsive to public opinion than the state governments would be.
Antifederalists, then, were the people who opposed the ratification of the Constitution because they preferred a weak central government and strong state governments.