what is the functionality of “.setCallback()” method in Salesforce Lightning ?

setCallback (Object scope,function callback,String name)

Sets the callback function that is executed after the server-side action returns. Call a server-side action from a client-side controller using callback.

Parameters
scope : Object The scope in which the function is executed. You almost always want to set scope to the keyword this.
callback : function The callback function to run when the server-side action completes.
name : String The action state that the callback is associated with.