According to the GitHub repository for the project, the listed features of the API are:
- Very fast: It runs efficiently even on phones. 60 fps on my 2-year-old phone!
- Open source: It is completely open source and free of charge!
- Standards: It works on any phone with webgl and webrtc.
One reason this is an amazing accomplishment is that, historically, web-based 3D is not fast. While people have been trying 3D in web browsers since the mid-'90s with VRML, it is only in recent years that processors have sped up enough, and that the code has become optimized enough, to make web-based 3D experiences that were more than very basic objects.
For more information about AR.js, make sure to check out the GitHub page, as it has a ton of details for you to get started with. Also, keep an eye on Etienne's YouTube channel for examples of what can be done.