The microcontroller is synchronous logic - cannot do anything without a clock signal.
This is almost always the case with processors. Parts of the chip have some asynchronous
logic (for instance some forms of interrupt are asynchronous, and the reset pin), but in
general everything is clocked.
The ATmega chips have several clock options, an internal RC clock, external clock
input or an inbuilt crystal oscillator circuit (which is normally what it used with
an external crystal or resonator).