In physics and chemistry, vacuum refers to a space without matter - a space full of nothing. There is no air, no molecules and no living beings. In colloquial language, vacuum is known as an airless space. By vacuuming, and thereby oxygen removal, food can be made long-lasting.
Explanation for children: A vacuum is a space full of nothing, there is no air, no molecules and no living beings. The universe is almost a vacuum, but even there there are still a few particles flying around. In a broader sense, vacuum can also be called empty space. For example, you can vacuum food, a machine removes oxygen from it and the food lasts longer.