Complex sentence: Beneficial alleles increase the fitness (i.e. the reproductive success of an organism in a particular environment) of an organism, whereas deleterious alleles decrease the organism's fitness and neutral alleles have no effect on the fitness of the organism.
Split into simpler sentences: Beneficial alleles increase the fitness (i.e. the reproductive success of an organism in a particular environment) of an organism, whereas deleterious alleles decrease the organism's fitness. Neutral alleles are neither beneficial or deleterious and have no effect on the fitness of the organism.