basically the low,normal, high, above, realtime
are just priority levels from lowest to highest order, so the higher the priority, the more attention that program will get when competing with others.
you should almost always avoid Realtime, as it takes up much too much resources of your computer and can make it unresponsive; definitely do not use this mode unless you have good reason to.
below and above are new and not very useful, they are kind of in the middle levels.
the most useful are Low, Normal, and High, these are really the only 3 you need to worry about:
By default programs run at Normal priority.
When process tamer tames an app it sets it to low priority, so it will get less cpu than others.
You might want to set your CD/DVD writer to a rule to force it to high priority to insure it gets the attention it needs.