What does it mean to quarantine a city?

Question

and why do it

Answer 1

In the US, power to quarantine a city resides with the state government, and therefore specifics vary by state. The CDC has a draft of a model law for the states, the Model State Emergency Health Powers Act, which it commissioned to provide a template for states' quarantine laws. The MSEHPA gives very broad powers in the event of a Public Health Emergency, including mobilization of militia for enforcement, closures of buildings and roads, control of hospitals, rationing, enforced vaccination or mandatory isolation, enforced treatment or mandatory isolation. The state is immune from liability in its actions.

Federal guidelines do direct which infectious diseases that are currently specified (by Executive Order) to be quarantinable. These diseases are cholera, diphtheria, infectious tuberculosis, plague, smallpox, yellow fever, viral hemorrhagic fevers, SARS, and new types of influenza that could cause a pandemic.  

Answer 2

what tinman said, and the only reason the government would have a city quarantine if there's an highly contagious (viral disease) like ebola or any type of deadly disease's  

Answer 3

No one goes in, no one comes out. All roads blocked. All persons must be accounted for. Only government officals allowed in and out.