The Earth is protected from the harsh conditions of space by a thin layer of gases that we call our atmosphere. The farther way from Earth's surface you go, the less dense (thinner) the air gets. Eventually, the air gets so thin that scientists consider it the "vacuum" of space (vacuum simply refers to empty space, containing no matter). This transition from atmosphere to space is found about 62-75 miles above the surface, depending on which scientist you ask. When spacecraft re-enter the Earth's atmosphere, they start to "feel" the effects of the atmosphere around 75 miles above the surface, so that's the figure I like to use.