Mixins are variable for the whole declaration block. Mixins are case sensitive. Mixins scope is same as variable scope. In other words if you have make a class in css then you can call that class in other class. Let me explain what I mean. Please read my previous article What is lessJS if you are new to LessJS.
LESS stands for Leaner CSS. Less is a pre-processor. Less extends css language. In less we can declare variable. Less runs inside nodes. In other word less is css framework which extends our css, it add more feature like initializing variable, mixings, functions and many other technology that allow to make css that is more maintainable.