real-time comments are the display of comments on a view of a post (typically on its permalink) as they are received by the post (presumably via webmention), without needing any explicit user reloading, refreshing, or any other tapping/clicking etc.
Real-time comments are one of several real-time interactions.
Real-time comments are a silo innovation.
Facebook was perhaps the first silo to implement real-time comments since 2012(?) at least, on both post permalinks, and sometimes on posts in a stream or in a collection.
Twitter, as of their mid-2014 front-end redesign, now supports (sometimes) showing real-time comments on tweet permalinks: