Imagine an odometer of a car. There are eight spaces each of which can display a one-digit number. Therefore, the highest number that can be represented is 99'999'999, after which the odometer turns to 00'000'000 again.

The same thing can happen in a computer. Every number has a fixed amount of bits it can use to store its value. As soon as all these bits are set, increasing the number again will cause an overflow, meaning that the number will wrap around to its smallest value.

This behavior has led to notorious bugs, such as the software of the Ariane 5 rocket malfunctioning due to an unexpected overflow. This caused a crash shortly after takeoff, costing several hundred million dollars.