The adverb anywhere means "in any location," or "wherever." If you're allowed to pitch your tent anywhere in the campground, it means you can choose any spot you like.
When you say, "I can't find the cat anywhere!" it means you've looked every single place you can think of, and you still haven't located your pet. If you win the lottery, you may be overwhelmed by the idea that you can afford to travel anywhere — you can visit any country in the world. English speakers started saying anywhere in the 14th century; earlier, they said owhere or oughwhere.