You have a notebook. On the first page, you put a table of contents. As you fill in pages, you note them down in the table of contents at the start.

When you want to delete a line, instead of erasing the whole page now (there are hundreds free still, why waste the effort), you erase the entry in the table of contents.

Now if someone finds your notebook, according to the table of contents there is no file at page X. But if they were to look through every single page, they would be able to find the page eventually.

This is loosely how file systems work. You can’t really use it to boost storage, the number of pages is finite, and if you need to write a new page, anything not listed in the contents is fair game to be overwritten.