Set your peferences with proity for instance 300 max 300 min when screen is locked is a good setting so your cpu uses less battery. Also, at lower battery or when when cpu is "hot" you may can have you CPU max lower.

 

I believe JDunc was talking about the profile settings, while the OP is asking about the sliders on the initial screen. OP, the max will be the maximum frequency that the phone's CPU will achieve. The min is the minimum it will go to. Typically, you'd want max to be at the highest value possible and min at the lowest. The phone has multiple speed "profiles" built into it and will scale speeds (between 300/600/800/1000MHz, by default) depending on CPU load. In other words, if you're doing something that requires CPU (ie. playing a game), it will crank up as high as possible. If you're doing something that doesn't require CPU (ie. idle phone with the screen off), it will turn down the CPU speed to save battery.

Without SetCPU, the phone will do this automatically. With SetCPU, you can set very specific times for certain speeds to occur, which would be done through the profiles that JDunc was referencing.