Property binding in Angular helps you set values for properties of HTML elements or directives.
property binding用于给html元素或者Directive的属性设置值。
The metadata for a component tells Angular where to get the major building blocks that it needs to create and present the component and its view. In particular, it associates a template with the component, either directly with inline code, or by reference. Together, the component and its template describe a view.
Component同template一起，描述一个view.
directives: apply app logic to what gets displayed.
Without a framework, you would be responsible for pushing data values into the HTML controls and turning user responses into actions and value updates. Writing such push and pull logic by hand is tedious, error-prone, and a nightmare to read, as any experienced front-end JavaScript programmer can attest.
