Natural selection is the evolutionary process in which organisms with beneficial inherited traits leave more offspring. Over generations, those traits can become more common in the population.
The process requires variation among individuals, some heritability of that variation, and differences in survival or reproduction. Selection acts on individuals, but evolution is measured as a change in inherited traits or allele frequencies across populations.
Natural selection does not mean that organisms consciously adapt, nor does it always produce perfection. Environments change, traits can involve trade-offs, and chance processes such as genetic drift also influence evolution.