To enable Photon, click on “Activate” for first time when using it.
It instantly is activated, same can be verified by clicking on any of the blog post images, the address for that will now be directly from WordPress CDN and not the local blog address.
This speeds up the blog load time since heavy images are directly served from WordPress CDN (which is a distributed infrastructure / cloud) as compared to loading all these images from a single server as before (more http requests hitting a single server etc etc).
Do try it out.
WordPress is a powerful content management system (CMS). Here is a useful resource about how WordPress works in a nutshell which is applicable for developers working with WordPress as well as users.