What is a binary overflow?

Overflow occurs when the magnitude of a number exceeds the range allowed by the size of the bit field. The sum of two identically-signed numbers may very well exceed the range of the bit field of those two numbers, and so in this case overflow is a possibility.

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And another question, what is unsigned overflow?

Unsigned Overflo. Which means the represented number is greater than the available bits or biggest numbers FFH for 8-bits and FFFFH for 16 bits. During subtraction the unsigned overflow occurs when there is a borrow into the MSB. In this case the correct answer is actually smaller than 0. In respect to this, how can overflow be prevented? Because integer overflows occur only for specific operand values in otherwise valid code, the only reliable way to prevent them is to use overflow checks or value sanity testing for every integer operation where an overflowing value could theoretically appear.