It’s a bit like an [analog odometer](https://editorial.pxcrush.net/carsales/general/editorial/odometer-day-102.jpg?width=1024&height=683). Calculators and computers only have a finite amount of bytes to store numbers in. On an odometer once you hit the highest number it can display (like 9999 on a 4-digit odometer), the next number will wrap it around back to 0000.
It gets a little bit more complicated when you consider negative or floating point numbers, but the basic idea still holds.
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