What is backend and frontend programming?
Frontend is what you do that the user can see. Like designing a user interface.
Backend programming is the code that the user doesn't 'see'. This is what works with the data behind the scenes. For example fetching/inserting/deleting/updating a database.
This website goes more in depth from a frontend and backend developer prospective.
In terms of Java EE webapplications, backend programming often concerns real Java classes in the model and controller parts (which the enduser doesn't directly see): Servlets, Filters, domain objects, database access objects (DAO), etc.. and frontend programming often concerns the view and user interface part (which the enduser directly see and interact with): JSP, HTML, CSS, JavaScript, etc.
Back-end is programming that the user doesn't see, in a web application it would be the server-side code that runs on the web server to enact the application's logic such as storing data to a database, verifying user registration info, and so on... the front-end is the part that the user sees and interacts with, i.e. the "user interface", in websites this is javascript and HTML (though thats just a presentation language, not a programming language) and also some parts of the server side language, such as PHP, could be considered front-end as they are used to dynamically generate the front-end page for the user.
Generally speaking, the "front end" is the user interface, and the "back end" is the code supporting that front end (responsible for database access, business logic etc).
This can be applied more generally though - you could consider a web service to have a "front end" which is the part which advertised the available functionality, then calls into the "back end" which does the actual work. You might even have a chain of services, where the front end of each service calls into its own back end, and then that back end can call into the front end of another service. Each service can then keep a consistent front end API over time, while changing the back end and how the front end talks to it. (Often this means keeping the front end fairly thin, but that's not always the case.)
In simple term, application front end is what you see (i.e the user interface) and application back end is the application engine that you do not see.
In client/server applications, the client part of the program is often called the front end and the server part is called the back end.