What is a mixin and why would I use one?

I see the mixin option on my business objects, but what can it be used for?

You can think of a mixin as a set of properties and/or behaviors that you want to apply to several other business objects. A simple example would be a Last Modified mixin that has a lastModified property and an Set Last Modified behavior that sets the value before update. You cod then apply this mixin to every business object and they will automatically have that property and behavior. You can read more about mixins at Apex Designer Docs.