If at anytime I get a little carried away with technical colloquialisms (a.k.a. geek-speak) please let me know. I don’t know who might be reading this and if you are not an IT professional, or are just starting out, you may not understand developer speak.
So, front-end. Simply, the interface through which you interact with your program or website. That could be the web pages themselves, though it would more likely be a form on a website that has text boxes and lists that you enter data through and submit.
The software package you use for your accounting has forms / windows that you use to enter and edit data etc. These forms are referred to by software and application developers as the front-end.