Historical versions with descriptive comments, courtesy of Frank Lee.
If you want to use comments to describe the work you did for a version, you'll want to check out the file, edit it, save it, and then check it in. When you check the file in, you can then add comments describing the work you did.
This is what Frank Lee did with this file three times. Each time, he checked the file out, edited and saved it, and then checked it in. Each time he checked it in, he added a comment. You can see those comments on the right-hand side of this picture.
He could have simply edited the file, saved it, and closed it — versions still would have been created for him. But in that case, he couldn't have added comments. Comments can be added only when you check out and check in a file.