Look at any group of the same species, such as the dogs in a park or the daisies on a lawn, and no two individuals are exactly the same. The differences between individuals in a population are called variation. Some of it comes from the environment, but there is usually extensive genetic variation within a population of a species.
Genetic variation means that individuals carry different alleles of many genes, so they have different genotypes and often different phenotypes. A population is all the members of one species living in the same area. Sexual reproduction mixes alleles from two parents, so the offspring differ from each other and from both parents.
The original source of new alleles is mutation, a random change in the DNA. This large store of genetic variation matters because it means that some individuals may be better suited if the environment changes.