A cave worked as somewhere to live many, many years ago. Then someone saw the possibility in stretching animal hides over branches to make a more mobile home. Eventually someone realised they could stack stones together to build walls, and so get the strength of a cave but built where you wanted it. Zip forward a few thousand years and a man is launched into space in a sort of house made from metal, propelled by a rocket.
Design is a simple thing that we can all use. Creative thinking for a practical purpose. Being able to imagine a stone as a spearhead, a cave as a house or a rock as part of a wall. It's a bridge between what something is and what it could be. Between the things we make and how we use them. It's not the whole story of the world we make around us, but it's an important part. It's always been there, helping us progress, and making things work better.