A population is a group of organisms of the same species living in the same area. Look closely at any population and the individuals are not identical. Rabbits in one field differ in fur colour, body size and speed. These differences between individuals are called variation.
Some variation is caused by the environment, such as a plant growing taller in a sunny spot. Much of it is genetic variation: individuals have different forms of genes, called alleles, so they inherit different characteristics. The set of characteristics an organism shows is its phenotype. There is usually extensive genetic variation within a population of a species.
Genetic variation comes from two main sources. In sexual reproduction, offspring receive a mixture of alleles from two parents, so they are different from their parents and from each other. Mutations also create new alleles. A population of clones would show almost no genetic variation, which is why variation matters for natural selection: it provides the different phenotypes for the environment to select between.