Welcome to DC and the forums!
While I suppose this is in the wrong place in the forum, I guess I'll try to answer you:
Force High means that Process Tamer will tell a specified app to run in High priority. I don't know much about hardware, CPU, etc. but I think this means that an application will use more of the CPU power which might lead to it responding quicker among other things.
Force Normal can be used if an app has a higher or lower CPU priority than you want it to. I use it for KatMouse utility. It is originally a High priority, but I don't need it to, so it's forced Normal
Force Low: I think you understand this now, but essentially it's the opposite of Force High. Maybe an app uses too much CPU time at the cost of other apps, so you can let Process Tamer automatically force it to use Low CPU.
Hope that made sense!