Is it something we just made up? Or is it something that was discovered?
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The current definition of a second is “a very large number times the time it takes for a specific atom to vibrate in a specific way under specified conditions”. The reason for this definition is that that’s the most precise window of time we can measure, in the form of atomic clocks. The exact number in this definition (9,192,631,770 vibrations) was chosen to match up with earlier definitions, which were based on older ways of keeping time.
Atomic clocks are really *really* accurate – the most accurate one [would be off by less than a minute if you used it to time the time since the beginning of the Universe](https://en.wikipedia.org/wiki/Atomic_clock) – so this definition is pretty precise.
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