The Anti-federalists feared a strong central government and the power that it would wield. Thomas Jefferson and James Madison were prominent anti-federalists. The anti-federalists were instrumental in getting the Bill of Rights passed which guaranteed basic freedoms like freedom of speech and freedom of religion. The most important amendment, however, to the anti-federalists was the Tenth Amendment which gave any power not listed in the Constitution to the states. This mention of states' rights was important to the anti-federalists because they feared that the states would ultimately dissolve and in time the federal government would become too large and impersonal--this was the colonial complaint against Parliament during the Revolution. This Tenth Amendment has been invoked many times when state leaders have not agreed with federal action and Jefferson and Madison went on to draft the Virginia and Kentucky Resolutions in which they praised the power of the states.
Anti-federalists were a group of colonists concerned a strong central government would one day overwhelm the populace and become tyrannical. The group had several well known and respected leaders such as Patrick Henry and Samuel Adams. The group played an important role in the ratification of the Constitution. They were opposed to it and attempted to stop it infamously in Pennsylvania. To prevent a quorum the anti-federalists refused to attend the state assembly. They were found, forced to the state house and locked inside to complete the vote.
When the amendments to the Constitution were proposed, the anti-federalists saw this as a chance to achieve some victory despite their defeat. The anti-federalists and Thomas Jefferson saw the amendments as a way to limit the power of the government. The Bill of Rights was the first move to satisfy the anti-federalists and assuage the fears of many the power of the government might grow too big. The most important amendment was the Tenth Amendment which limited the power of the federal government. The amendment provides the powers not delegated to the federal government would be retained by the states. The anti-federalists felt this was essential to ensure the states bound together would always be more powerful than the federal government.