One of the core principles of liberal democracy is that of limited government: the role of government should be limited by checks and balances and a separation of powers, because of the corrupting nature of power. In the context of the US, limited government means that the power of the federal government is subject to limitations as laid out in the Constitution, so that it cannot simply impose its policy on the state and its citizens.
The Bill of Rights also prevents the federal government from restricting the rights of the individual or the rights of states. Amendments such as the 1st (freedom of expression) and 4th (freedom from unreasonable searches) can be seen as limiting government by protecting individual freedoms. The 10th amendment is clearly designed to protect the power of the states by stating that any power not given to the federal government is reserved for the states or the people.
Modern conceptions of limited government cover the extent to which the federal government plays a role in social and economic policy. For conservatives, and particularly for libertarians, there is a desire to reduce government involvement in areas such as expenditure (for example, on welfare provision). Social programmes are frowned on as being part of a 'big government' agenda, which is rejected by many in the US.