Prescaler
Clocks included internally or on board (external) are sometimes too fast for a particular task on hand. Let’s say you want to blink an LED and the clock is running at 16Mhz. The clock will tick 16 million times per second. This means that the main loop will execute 16 million times per second. You could keep a counter and blink only once per million seconds, but this might not be the best solution in constrained environments like the ATMEL Processors which only have 16 bits data types.
So we use a prescaler to tune down the clock speed. The prescaler divides the clock speed to get to a manageable speed.
Example: 16mhz clock / 4