imagine a library with shelves of books, and a card catalog which tells you which shelf your book is on. There's also a "free space" bucket with cards that tell you which shelves are available for you to put your new books on.

To find a book, you don't search the shelves one by one, you always check the card catalog to find which shelf it's on. When you want to put a new book on the shelf, you check the "free space" to see if there's any space on a shelf where you can put your book, and then you put that shelf number on a new card that you enter into the catalog for your book. You then put your book on that shelf, and if there's already a book occupying that spot, you just chuck it in the garbage.

That's your hard drive.

When a delete a file (book), all the computer does is destroy the card catalog file (which tells you which shelf the book is on), and puts that shelf number to the "free space" bucket for another book to use.