A controlled or prescribed burn deliberately sets fire to a defined area of vegetation under carefully managed conditions, reducing the accumulated fuel load that would otherwise make a future, uncontrolled fire considerably more intense and considerably harder to manage once it eventually and inevitably occurs.
Many ecosystems have actually evolved alongside regular fire, with certain native seeds requiring the heat or smoke of a burn to germinate at all, which means an area protected from fire entirely for too long can paradoxically become less healthy, not more, than one managed with periodic, carefully controlled burning.
Planning a burn requires careful attention to weather, moisture, and wind conditions, since the same fire that safely clears accumulated fuel under the right conditions can become genuinely dangerous under slightly different ones, which is why the practice depends heavily on trained expertise and careful, patient timing. That gradual build tends to be the whole of the method, in the end.