Mixins are a way of reusing a class’s code in different class hierarchies. For example, you might have a class called Employee which has methods like clockIn. The code in those classes may be useful for both Bartender and Nurse. But, now imagine you're introducing a class called Doctor. You probably have some functionality on Nurse (such as a method called takeTemperature) that you'd like on Doctor, but not that you want to add to Employee. This is a great place to use a mixin called Medical.

class Employee {
  void clockIn() {...}
}
mixin Medical {
   int takeTemperature {...}
}
// use mixins on your classes via the `with` keyword
class Nurse extends Employee with Medical {}
class Doctor extends Employee with Medical {
  performSurgery() {...}  
}
class Bartender extends Employee {}

With this architecture, your bartender can 'clock in', but cannot 'takeTemperature'. Both Nurse and Doctor can do both, and the doctor can performSurgery.