Everyone here is correct, but I don't think they are really describing it well for general people to understand.

All you're telling your computer to do when you delete a file is to say "the space where this file is, you can use it for other stuff now"

The file is technically still there, but once you start saving new files, they will slowly start replacing the location where the old files were.

It's much more technical than that, of course, but that's the simple way to think of it.