What does dirty side mean in Formula 1?

Answer 1

In F1, drivers generally tend to follow a racing line on a particular track. There is also a specific racing line at the start-finish stretch and just before the first turn. The dirty part of the track on this stretch, is the area of the track which is NOT the racing line. It is known as the "dirty part" because over the course of the race, a layer of burnt rubber from tyres is laid over the racing line, which provides more grip to cars and is also relatively free of any kind of dust or loose particles.

At certain circuits, drivers do tend to lose track positions when starting from the "dirty part of the track" as the cleaner area provides better grip and thus accounts for quicker acceleration when the five red lights go off.