Genes are small proteins that decode what we inherited from our ancestors, and they partly shape everything we are and do. Each individual has a fixed set of genes, but the set of genes varies between individuals. This variation makes it useful for us to talk about genetic composition.
That is, which genes exist in an individual, and how common are they if we look at all individuals of a species (or a local population of a species). If the genetic composition changes, evolution may happen.