Basically it restores the Registry to an earlier date of your choosing. In other words, you install something that screws up your system. It was working OK yesterday, you you restore to "yesterday" and your system is like it was with the new ly installed garbage not registered. You can then delete it manually and you are back in business. I have used it several times on ME and XP . . . it works well.
You can set the space limit it consumes, and when it is full, it starts "rolling over."
I find it a very worthwhile feature.