It comes from an Italian economist Vilfredo Pareto, and it’s sometimes known as the Pareto Principle in his name. He came to the conclusion after analysing the distribution of wealth in Italy, and finding that 20% of the population held 80% of the wealth. When he looked further he found that the same ratio held for wealth distribution in other countries.
The rule points to a disparity between input and outcome, but in fact it doesn’t have to be 80-20; years ago in another job I used it to analyse the product portfolio of my clients and point out where they were at risk 80 % of revenue from 20% of products was already risky, the strongest companies were closer to 65% from their top 20% products. And there you see that it doesn’t have to add up to 100, just that as decimal creatures a 100 total is the most satisfying.