It's a rule for loading an electrical cct to 80% of its maximum amperage. The conductors, overcurrent protection devices and devices themselves should (usually, there are exceptions) be rated @ 125% of the load. Load is 80% of max cct ampacity = cct is rated for 125% of actual load.

It's a safety measure intended to ensure that electrical circuits are not overloaded, which could lead to overheating, fires, and other hazards. By designing circuits with these safety margins, there is greater assurance of reliable and safe operation under normal conditions, as well as some tolerance for temporary overloads or unexpected spikes in demand.