It's like a book. The start of the book has a table of content, that lists all the chapters in the book. If you would want to remove a chapter, you could remove the entry to that chapter from the Table of Contents.

So, effectively, not much is removed, only the entry in the Table of Contents is gone, so you cannot easily find the chapter again. It is still possible, but takes a lot more efffort (skipping through all pages). You could even overwrite the pages of that chapter with something else, adding an entirely different chapter, as long as the chapter is of the same size or smaller than the one you just removed.

Same happens with files on computer storage. There is an index of where all files are located. By removing a file, the entry in the index is removed, effectively making it a lot harder to find that file again, and even allowing other files to be written on the same spot previously filled with that file.