When one or more individuals are equally satisfied with the outcome of a situation or objective. For example, if you washed my car and I did not have money to pay you, but offered to wash your car the next time it was dirty ----You could respond by saying "fair enough". In your sentence the sender is asking "Would you mind coming in at eight o'clock" and the receiver responds by saying "fair enough". The receiver responds by saying fair enough because he or she is satisfied/agree/or does not have an issue with coming in at eight o'clock.