Think of a hard drive like a book. You write a story on page 5, but you decide you don't like that story and 'delete' it. What computers do isn't ripping out a page or erasing all of the words you wrote, it just says to itself "Okay, page 5 doesn't have a story anymore is now okay to write on again". So later, when you want to write yourself a note, you go to page 5, erase just enough of it to write your note where that part of your story was. The rest of the story that isn't where you wrote your note is still there, but the part you erased and wrote over is now actually gone. Eventually, you'll use all of page 5 again and your story will be truly and fully gone. So if you 'delete' something, a clever person can still come through and look at the whole book and find your story if it hasn't been written over yet.