The SI unit of time is the second. When you think about it, the unit of time used thousands of years ago would have based on astronomical observations such as the phase of the moon or position of the Sun.
In fact, the second was originally defined as 1/60 of a minute, which is 1/60 of an hour which is 1/24 of the mean solar day. Multiplying those numbers makes the second 1/86,400 of the mean solar day.
Today, the second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. In everyday language, we say that it's based on the frequency of light given off from a cesium 133 atom as it makes a transition between certain energy levels.
We'll discuss the fact that atoms give off light in a later topic.