Can someone give me a simple definition of what a government shutdown is?

a government shutdownoccurs when Congress fails to pass sufficient appropriation bills or continuing resolutions to fund federal government operations and agencies, or when the President refuses to sign into law such bills or resolutions. In such cases, the current interpretation of the Antideficiency Act requires that the federal government begin a "shutdown" of the affected activities involving the furlough of non-essential personnel and curtailment of agency activities and services. Essential employees are still required to work without pay until the government reopens, when they may then receive back pay. These employees may include medical professionals in the Veterans Hospitals and TSA agents.