The Mean and Standard Deviation of the population's unfitness are statistical measures that give you information about the distribution of the unfitnesses of the individuals in your population. The mean unfitness is simply the average unfitness of all individuals. It gives you a feel for how good or bad your population is overall. (A low unfitness is better.) The standard deviation gives you a feel for how much genetic diversity there is in the population. If it is very low, most of your individuals are likely to be quite similar. It may then be time to change some genetic engine or agent settings or save your population and start a new run.