Till now Facebook comments used show whatever you type in there with keyboard. The comments have got smarter now with the recent improvement added by Facebook development team. Its very often you want to enter an URL in the comment to point some other information available on related topic. Others would have to click that URL or copy paste in another tab to get an idea what the link is about.
Facebook recently added this new feature so that when you add an URL in a comment, it gets detected automatically. That means, Facebook processes the comment before posting to find an URL in it and then pulls the webpage with that URL in the background. Once the entire webpage is pulled, it gets processed further to create a small preview of that page. The preview contains set of images, videos and textual summery. You get a chance to select an image/video or alter textual description before comment is posted on a wall.
This a very handy feature and may sound ordinary now but can help a lot to decide whether to open the webpage or does the webpage have what you wanted.