Augmented reality is using electronic devices to add information to the world (reality) around us.  Wikipedia has a good description:  https://en.wikipedia.org/wiki/Augmented_reality  

AR is one piece of the evolution from the flat World Wide Web to the Spatial Web, which enhances connected websites with people, spaces, and assets.

There are many ways to implement AR. You've seen it buying furniture or art, placing an object or painting in your room, on your wall. Instagram has AR inserted into some posts. When you use an avatar in video conferences it is also AR.

Winchester Riverwalk uses an app called Hoverlay that runs on your mobile device or tablet ,  and overlays the AR content on the screen image (captured from the camera) with images, text, video, 3d objects, and links that can provide additional information from the internet. 

Hoverlay has a YouTube channel with examples and discussions about AR. 

Other forms of AR can use goggles like https://www.apple.com/apple-vision-pro/  or display screens attached to your glasses https://en.wikipedia.org/wiki/Google_Glass