Directive 80/181/EEC is a rulebook made by a group of grown-ups called the European Union. It says that all the countries in the European Union should use the same rules for measuring things like weight, length, and volume.
For example, if you go to France and buy a bag of apples, they will tell you how much the apples weigh in their system called "grams". But if you go to Germany and buy the same bag of apples, they will also tell you how much the apples weigh, but in their system called "kilograms".
This can be confusing if you're trying to compare prices or recipes between countries. So, Directive 80/181/EEC makes sure that everyone uses the same system, so it's easier to understand and compare things in different countries.
It's like when you and your friends are playing a game, and everyone has to play by the same rules, otherwise it's not fair. Directive 80/181/EEC just makes sure that all the grown-ups in the EU are playing by the same rules for measuring things.