The best way for me to understand the terms conceptually is to Imagine a Website as a restaurant.

Front End: The front end is what you as a user sees on the website, much like in the customer section of the restaurant. The atmosphere, the menu, the wait staff, how you order food and how food goes to your table. the counter etc... Just like the designs of the websites, the buttons, the pages, how you navigate the website, the interfaces, the functionalities you have as a user etc... These are all front end.

Back End: Much like the restaurant, these are things that users dont see on the websites. The kitchen, the storeroom, the fridges, the stock purchasing process, the account spreadsheet of he restaurant and all the other  things customers will not know about but is absolutely crucial to the restaurant's operation. So for websites these are the database, the API calls, all the other things.

Hope my analogy is good enough!!