The Apex Controller is a server-side programming element that allows us to retrieve account data from the Salesforce platform and, in the context of the framework of our Search First Before You Create Lightning Component, is described by Salesforce as follows:
"The framework supports client-side and server-side controllers. An event is always wired to a client-side controller action, which can in turn call a server-side controller action.
For example, a client-side controller might handle an event and call a server-side controller action to persist a record.
Server-side actions need to make a round trip, from the client to the server and back again, so they usually complete more slowly than client-side actions."
For example, a client-side controller might handle an event and call a server-side controller action to persist a record.
Server-side actions need to make a round trip, from the client to the server and back again, so they usually complete more slowly than client-side actions."
Reference:
Lightning Components Developer Guide: Creating an Apex Server-Side Controller
https://developer.salesforce.com/docs/atlas.en-us.lightning.meta/lightning/controllers_server_apex_create.htm
Lightning Components Developer Guide: Creating an Apex Server-Side Controller
https://developer.salesforce.com/docs/atlas.en-us.lightning.meta/lightning/controllers_server_apex_create.htm