Quarantine is enforced isolation, typically to contain the spread of something considered dangerous (often disease).
For example, due to the risk of introducing rabies from Continental Europe, the United Kingdom used to require all dogs (and, indeed, most animals) introduced to the country to spend six months in quarantine at an HM Customs and Excise pound; this policy was abolished at the beginning of the 21st Century in favour of a scheme generally known as Pet Passports, where animals can avoid quarantine if they have documentation showing they are up to date on their appropriate vaccinations.
In the case of people, quarantine usually raises questions of civil rights, especially in cases of long confinement or segregation from society.
The first astronauts to visit the Moon were quarantined upon their return at a specially built Lunar Receiving Laboratory.
Some quarantine periods can be very short, such as in the case of a suspected anthrax attack, in which persons are allowed to leave as soon as they shed their potentially contaminated garments and undergo a decontamination shower.
The purpose of such quarantine-for-decontamination is to prevent the spread of contamination, and to contain the contamination such that others are not put at risk from a person fleeing a scene where contamination is suspect.