Hi Andrea,
The markers a shaped like a fold of paper, with its insides being lined with magnetic strips which are attracted to each other and therefore hold fast to the top of a page and don't fall off.
They're quite handy. I have a set of Batman ones at home for some of the books I'm reading. They don't damage the pages and are a novel way of finding where you were last reading.
Just in case that didn't come across clearly, I've added a picture of a single one, which will give you a much better idea of what they look like and how they work.
Thank you for using the Q&A Forum.
Aaron