It's used just to assert properties of a function, for example if you have a function `rot13`, you could say "for all string `input`, input == rot13(rot13(input

It's used just to assert properties of a function, for example if you have a function `rot13`, you could say "for all string `input`, input == rot13(rot13(input))". And the test library will generate that input string randomly (or with a seed if you have a failing case you want to put in a regression suite), and if it finds a failing case it will search for the minimal input string which also fails.