What, exactly, does ‘containment’ mean on a wildfire?

Perhaps no word is more misunderstood and misused in discussions of wildfires – even by firefighters themselves – than the word “containment.”
Simply put, the containment percentage is the amount of the fire area that is surrounded by a line that officials believe the flames will not jump.
“To say a fire is 100 percent contained is to say we have a spherical line such as rocks, roads, streets or fire line where there is no combustible material,” said Eric Sher