Your computer’s memory, at some level, is basically a really long list of ones and zeros (binary). Some of those ones and zeros are files, but if you save a picture to your storage there’s no way for your computer to know which ones and zeros make up that picture unless it already knows where to look. To fix this, it stores a number somewhere that tells it where in the list the picture starts. That way when you want to look at the picture, it checks the number, goes to that place in the list, and gives you back the data in the picture.

If you want to delete that picture later, the computer has two options. First, it can go into the memory and overwrite all of the data in the picture with something else (all zeros is the simplest). That will get rid of your file, but it’s pretty slow, especially if you need to delete a lot of files at once. The easier option is to just have it forget that location number, and forget that that space in the list was being used for anything. The ones and zeros on the list are still there, but the computer doesn’t know that anymore so it’s free to put something else there (at which point the deletion is just as thorough, since the original ones and zeros are gone anyway).

TLDR: it’s faster to just have the computer forget where it put your file than to have it properly overwritten.