CPU frequency
Your CPU's clockspeed ("how fast it is")
The higher, the faster and smoother your phone will run. Normally you gain more battery (and speed) by ajusting the governor thresholds and not the frequencies. Keep in mind that high(er) frequencies drain your battery faster, as the CPU works more ("harder") and needs a higher core voltage to operate properly. Thereby overclocking leads to more heat, which might damage your phone. Be warned! In reverse this means that your phone will run slower when underclocked, but it will use less energy, which results in a prolonged battery life. Clocking too low (<400MHz) may lead to your phone being locked in "sleep" mode - you cannot switch it on with a button-press anymore. This is one of the reasons, why clocking below that speed is available in "poweruser" mode only. If you're out to extend your battery uptime, you might want to have a look at other common battery drainers.