A Prescaler is a PCB mounted semiconductor that allows you to reduce your high frequencies into lower frequencies using an integer division technique. Prescalers will take the higher frequency, divide it by a value and output it to a timer, or to whatever the output is set too. Prescalers allow you to easily clock a timer at whatever rate you need it to be.
Prescalers can be used in higher frequency applications to extend the upper frequency to the likes of PLL's (Phase-locked loops) and frequency counters. In hindsight, prescalers are count-dividers and can be used accordingly.