Imagine that someone is reading a book in a chair. You stand behind the chair and start to read the book that this person is reading. In this situation, you're "reading over their shoulder".
You can use the phrase "reading over (someone's) shoulder" to talk about reading someone else's book, newspaper, magazine, etc. It doesn't matter where the reader is standing or sitting.
It's also possible to watch TV over someone's shoulder. If one person is watching a TV show alone, and you start to watch it while pretending to do something else, you're "watching over their shoulder".