Quarantine is keeping new fish (or sick fish) in a separate tank for a few weeks before adding them to your main tank, to avoid introducing disease to healthy stock.
A simple bare quarantine tank with a cycled sponge filter and a heater lets you observe new arrivals and treat problems without medicating — or risking — your whole display tank.
It is the single most effective way to prevent outbreaks, and the step most often skipped. Even a basic quarantine beats none.