In evolutionary terms, variation simply refers to the differences you see among individuals. In other words, individuals from any species aren't all the same. Look around a room full of people, and you notice that all of them look different. That's variation. You can see how heritable changes — hair color, eye color, facial structure, height, and so on — manifest themselves outwardly. But these changes also manifest themselves in ways that aren't so easy to see, such as propensity for certain illnesses or blood type. And if those differences are a result of genes, that's heritable, or genetic, variation.
Even though variation is a simple concept to grasp, it's a crucial component of evolution. Without heritable variation, evolution couldn't occur, because change couldn't occur from generation to generation. If every individual in one generation were the same, every individual in the next generation would be the same. Variation has to be present before natural selection occurs; you can't have the sorting-out process if nothing's available to sort.
You're most aware of small differences in things you spend a lot of time interacting with. Just because all hyenas (or wild dogs, ducks, or any other nonhuman organism) seem to you to look the same, act the same, and do the same things doesn't mean that those species lack variation.
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