Precise time measurements are essential to GPS. Synchronizing the satellite clocks within nanoseconds (billionth of a second) of each other makes it possible for a receiver to know its position on earth within a few meters.
Atomic clocks are quite complex, but the theory is simple.
They rely on the fact that atomic transitions have characteristic frequencies (think of the orange glow from the sodium in table salt if it is sprinkled on a flame).
Like all clocks, they make the same event happen over and over. For example, the pendulum in a grandfather clock swings back and forth at the same rate, and swings of the pendulum are counted to keep time. In a cesium clock, the transitions of the cesium atom as it moves back and forth between two energy levels are counted to keep time.
Cesium clocks can give time accurate to 1 nanosecond per day.