What is the 80 percent rule in cycling?

Could you please elaborate on the concept of the 80 percent rule in cycling? Is it a training principle that cyclists adhere to in order to optimize their performance? If so, how does it work and what are the key elements that cyclists need to consider when applying this rule? I'm particularly interested in understanding how it can help prevent overtraining and burnout, as well as its potential impact on overall fitness and endurance.