A vacuum is an empty space, which has no atmosphere nor anything. Our atmosphere applies pressure to our bodies (see atmosphere and G-force) , but in a vacuum there is no such pressure, which would make your body "explode", as the inside of our body is built to match the same amount of pressure on the outside. If those to things doesn't collapse, we either imlode or explode. As you probably know, there isn't the mixture of oxygen, CO2 etc. out in space. Instead, there's a vacuum.
Did you know that out in space, theres absolutely nothing? It's called a vacuum!