Pareto Principle in Software Testing
80/20 principle states that a small part of invested funds or efforts is responsible for a large portion of the derived products or earned rewards.
For example, to receive 80% of the results achieved in the work, you should spend 20% of the total time.
It turns out that in practice, 4 / 5 of your efforts (large proportion) do not have any relation to achieved results. This, incidentally, goes away with what people normally expect.
Applying Pareto principle to software testing, it would be correct to say that 80% of errors being concentrated in 20% of the developed product functionality. So errors in the software (modules, functional business processes, etc.) are distributed unevenly, and Pareto principle is working here.