The famous Kata Video Store from Martin Fowler's book Refactoring (https://www.amazon.com/Refactoring-Improving-Design-Existing-Code/dp/0201485672). Robert C. Martoin (Uncle Bob) also used this Kata in his Clean Coders Video as a Refactoring example (https://cleancoders.com/episode/clean-code-episode-3-sc-3-videostore).
Refactoring is changing the structure of code without changing it's behaviour. To ensure that we don't change the behaviour of our code, we use tests and test coverage after each code modification.