Sometimes, when adding two binary numbers we can end up with an extra digit that doesn’t fit. This is called an overflow error.
This sum is fine as the original numbers have two digits, and the result of the sum also has two digits.
This sum has an overflow error. The original numbers had two binary digits, but the answer is three binary digits long.
The effects of an overflow error can vary. It might make the program crash or it might just ignore the extra digit on the left and produce an unexpected result (in this case, 2 + 3 = 0!).