In a client-server application, backend code runs on a server. The frontend component, or client, is an interface on a local computer or workstation that and enables users to access the remote server and manipulate the backend code. A user cannot directly access backend code.
In Magento, customers interact with the storefront (UI), merchants interact with the Magento Admin (front end client / browser). These interactions launch the server-side "backend" code, or Magento application programs, remotely.