If we recall our observations, we see that the range of numbers in 8-bit 2's complement is -128 to +127. Thus, we get screwy answers when we add numbers that are too big and which cause the real sum to be larger than what we can represent. This is called overflow.

Notice that if we treated the above addition as unsigned numbers, everything is fine because 168 is 10101000, as long as we are looking at it as an unsigned number and not as a 2's complement number.