Why do people say you're welcome?

Why is it that, when someone thanks me for something, I often hear them say "you're welcome" in response? It seems like a common phrase, but I'm curious about its origins and the underlying meaning behind it. Is it just a polite way of acknowledging the thanks, or is there more to it? Does it vary across different cultures and languages? I'd love to learn more about the significance of this phrase and why it's so widely used.