A DVR is a "Digital Video Recorder." Traditionally, these are boxes that sit underneath your TV and allow you to record programs off your TV for watching later.
At api.video we allow instant rewind of live streams, and we call this our DVR feature. When a stream is live, you can rewind the live stream up to 5 minutes backwards to watch 'nearly live' content. Perhaps you missed what was said, and want to make sure you understand the speaker. Or perhaps there was a sports play that you'd like to watch again. OUr DVR feature allows your users to rewind the live stream and see the action again!
You can read more about this feature on the instant replay glossary page.