These photos require crossing your eyes! Basically your right eye is going to see the left image and the left eye is going to see the right image. It can be very tricky at first, you can first focus on being able to cross your eyes, then on changing the distance between the two images, then on matching up the two images and focusing. It’s worth it.
If you can’t do it then look at my Lenticulations instead!
To see a more complete list of stereographs, visit my flickr photostream!