We use timers every day – the simplest one can be found on your wrist. A simple clock will time the seconds, minutes and hours elapsed in a given day – or in the case of a twelve hour clock, since the last half-day. AVR timers do a similar job, measuring a given time interval.
An AVR timer in simplest term is a register. Timers generally have a resolution of 8 or 16 bits. So an 8 bit timer is 8 bits wide, and is capable of holding value within 0-255. But this register has a magical property – its value increases/decreases automatically at a predefined rate (supplied by user). This is the timer clock. And this operation does not need CPU’s intervention.