Fitness describes a reproductive success of an individual. For example, there are green and yellow beetles in a population. If a color affects the reproductive success, for example, green beetles will have more offspring that the brown ones because of their color. So, green beetles have higher fitness. And the beetles with decreased fitness will have less offspring.
So, if a mutation creates a new allele which decreases the fitness, the altered allele will not be inherited in such a ratio as the unaltered allele will.