What does it really mean when a wildfire has been ‘contained’ or ‘controlled’?

Firefighter updates on wildfires such as the devastating Camp and Woolsey fires in California, often include a containment figure, usually expressed as a percentage. But what does that actually mean?
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Firefighter updates on wildfires such as the devastating Camp and Woolsey fires in California, often include a containment figure, usually expressed as a percentage. But what does that actually mean?