I like this question.
Front-end: For a website this is what you see in the browser. For example when you came here to ask this question what you saw is the Front-end of musing.io. Front-end is mostly written in HTML, CSS, and Javascript. Front-end shows user input forms and raw data received from Back-end in a nicer understandable way.
Back-end: This where all request for contents are processed. All the data processing, complex calculations and storing are back-end's job. You can code a back-end in many language including PHP, Javascript (Node JS), Python, C++ etc. From back-end you can generate code for front-end along with user requested data. Many API is a good example of Back-end.
Interestingly with modern Javascript you can use the same programming skill in Frond-end and Back-end. Good luck with learning JS.
Thank you! It was in response to a previous question I asked about the first skills I should learn that someone suggested I learn JS as I could use it on the front-end and back-end.
Be prepared. There could be many more questions coming like this as I re-start my coding journey ;-)