From the VIIRS sensors (375 m resolution) aboard NASA's Suomi-NPP and NOAA-20 satellites, via the FIRMS program (Fire Information for Resource Management System). Files are regenerated roughly every hour and this site re-checks them every 5 minutes.
A 375 × 375 m pixel in which the infrared sensor spots a thermal anomaly. A large wildfire produces hundreds or thousands of detections on each satellite pass.
Fire Radiative Power, in megawatts. It is the standard measure of wildfire intensity: the higher the accumulated FRP of an area, the more violent the fire.
The satellites pass over every point on Earth about 4 times a day, and NASA's near-real-time processing adds 1–3 hours. A new fire can therefore take a few hours to show up.
Clouds or thick smoke can hide the ground, a fire under dense canopy may be invisible, and a small fire may have started between two satellite passes.
The Fire Weather Index computed by the Copernicus Emergency Management Service (ECMWF forecast): it estimates the meteorological fire risk (drought, wind, temperature), not ongoing fires.
Météo-France's official bulletin, published daily from June to September, rating every French département on a 4-level danger scale. It is the map shown on French TV weather reports.
The sources (NASA FIRMS, Copernicus, Météo-France) are open and reusable with attribution. This site is an information tool: it is in no way a substitute for official alerts. In an emergency, call your local emergency number (112 in Europe).