Revision, as the name suggests, is going back to an exercise or learning material you worked with some time before and check how much you remember.
For listening comprehension training this means when one or two weeks later you go back to some listening or video material that you worked on earlier (gist, scan, detail, study) and you check how much you understand of the same material now, what you can still understand clearly and what has been lost since the last time you heard.
This serves for various things.
Usually two weeks is enough time to “forget” whatever your short term memory would probably still remember for a couple of days. So if you check the same material after 2 weeks, you should get a fairly accurate picture of your present comprehension skills.
Whatever has been lost in terms of understanding is a good indicator of where to look again and do some extra work and practice.