What is evolution, in plain terms?

Evolution is change in the heritable characteristics of populations across generations. The mechanism Darwin identified, natural selection, is straightforward: individuals vary, some of that variation is inherited, more offspring are produced than survive, and individuals whose traits suit their environment tend to leave more descendants, so those traits become more common over time. Mutation supplies new variation, and other processes, genetic drift, gene flow between populations, sexual selection, also shift populations. Accumulate enough change and populations diverge into separate species. Evolution is a fact observed directly in the field and the lab, and evolutionary theory is the well-tested explanatory framework built around it, supported by fossils, comparative anatomy, embryology, and genomics that independently converge on the same family tree.

Two persistent misunderstandings are worth clearing. Individuals do not evolve, populations do, so no organism adapts during its lifetime in the evolutionary sense. And evolution has no goal, no ladder, and no drive toward complexity or toward humans, it is simply differential reproduction in a given environment, which means a trait beneficial today can become a liability when conditions change. Humans did not descend from modern apes; humans and other apes share common ancestors, which is a different claim. Evolution is also observable on human timescales in fast-reproducing organisms, which is exactly what antibiotic-resistant bacteria and pesticide-resistant insects are, populations evolving under selection pressure we applied, which makes evolutionary biology intensely practical for medicine and agriculture.