The white flag warns drivers of slow-moving vehicles on the track. This may be a slow-moving race car, a safety car, a medical car or other equipment.

In F1, the white flag is waved on the last corner at the end of free practice to signal cars practising their start from the grid

Other formats sometimes use the white flag, from the starters box, to signal the last lap. 

Andrew Burden

The author Kiwi F1 Fan