The Pareto Principle is simply described as 80% of the results, comes from 20% of the effort. The other 20% of the effort comes from 80% of the effort.
Vilfredo Pareto was the person who discovered this, when he realised that 80% of all property in Italy is owned by 20% of the population. He theorised that this rule would carry across multiple other fields.
The common application is business' being picky in what they focus on, and spend time working on. 80% of their profits will come from 20% effort spent in the right thing. The other 80% of their effort will only result in 20% of their profit.
Or an application for a solo software developer is, be careful what you work on, as you only need to spend 20% of your effort working on the right, high-impact job/activity for the majority of your success/achievements. Hard-work isn't all equal, and some hard-work can leave you no-better off than when you started. Or maybe even worse!