A wildfire is a large fire burning through trees, grass, or dry country, moving on its own and out of anyone's control. The word tells you two things at once: it is outdoors and in nature, and nobody is holding it back. A fire in a kitchen or a factory is never a wildfire, however big it is.

It can be countable or uncountable: "a wildfire started near the town", but also "wildfire season" and "the threat of wildfire". The word has become very common in news reports over the last twenty years, along with talk about hot, dry summers. American English uses wildfire most; British and Australian speakers often say bushfire or forest fire for the same thing, and Australians say bushfire almost always.

There is one use you will meet more often than the fire itself: the fixed phrase "spread like wildfire". It means news, a rumour, an illness, or a fashion moved incredibly fast from person to person. "The story spread like wildfire." This is a set phrase — say it exactly this way, and don't change "spread" to another verb.