As it pertains to equalizers the center frequency is the exact frequency to which the EQ filter is tuned. Equalizers are filters – at least analog ones are (most digital ones are programmed to emulate filter behavior). When you boost or cut an EQ frequency you are always changing the gain more than just that one frequency as determined by the Q of the filter circuit (or digital program). The center frequency is where the most change takes place (boost or cut), and the filter has progressively less effect on frequencies that are further and further away from the center frequency.