A vacuum is essentially a great lack of something — a place where virtually all matter, even air, has been removed. It can describe something that's literally empty — or something that just seems that way, like a political vacuum.
You probably have a vacuum at home. A humble household appliance named after a principle of physics, the vacuum is a device for sucking up dirt off the ground. It works by creating a vacuum — that is, it sucks all the air out of a particular place so that the surrounding atmospheric pressure will flood in, taking with it all the nacho chips, dog hair, toast crumbs, and whatever else you have lying around your house.