Clocks exist in many forms. Candles with marks on them to show how long they have been burning. Sundials that project the Sun's shadow onto graduated clock faces. Clockwork clocks that gradually release the energy stored in wound springs. Clocks that measure the vibrational frequency of crystals such as quartz, or even atoms.
Most clocks measure how many times a repetitive action is carried out.
These vibrations are counted by electronic circuits. We can also use light to make a clock, at least in theory. To do this we need to bounce a pulse of light between two mirrors that are a known distance apart. In other words, the round trip from one mirror to the other and back again will take the light pulse one second. Probably the most obvious one is the separation distance of the mirrors, but in reality we could put them very close together.