What is the 3 second rule in cycling?

Excuse me, could you elaborate on the so-called "3 second rule" in cycling? I'm curious to understand what it entails and how it is applied in the sport. Is it a safety guideline, a racing strategy, or perhaps something else entirely? Could you provide some context and perhaps explain how it affects riders and races? I'm genuinely interested in learning more about this rule and its significance in cycling.