$A namespace in javascript controller lightning cmp in Salesforce?

The $A namespace is the entry point for using the framework in JavaScript code.
For all the methods available in $A, see the JavaScript API at https://<myDomain&gt;.lightning.force.com/auradocs/reference.app, where <myDomain> is the name of your custom Salesforce domain.
A component bundle can contain JavaScript code in a client-side controller, helper, or renderer. Client-side controllers are the most commonly used of these JavaScript resources.