title: What is $model in Yii2?
body: <p>I'm new to MVC and Yii Framework. The $model variable seems very confusing to me.</p>
  
  <p>Where is it declared in the fist place? Where does it come from?</p>
  
  <p>When I work with GridView I see that some functions take $model as a parameter. Neither model nor model search of this GridView declares $model variable anywhere. Yet it is widely used in all sorts of data management. It just doesn't make sense to me.</p>
  
  <p>So I need a simple, straight forward, "for dummies" explanation of $model variable in Yii Framework v2. Help in clarifying these questions is much appreciated:
  1. What is the origin of $model variable?
  2. How to identify what model of the app is the $model variable representing when it's used in view files?
  3. There are sometimes multiple $model variables in a single view file. Do all of them represent one model class? How to distinguish them when used for multiple classes?</p>
  
  <p>Thanks.</p>