Air is a mixture of gases. This mixture envelops our earth; the envelope of air is called the atmosphere. Humans and animals need air in order to breathe. More precisely, they need the oxygen in the air.
Oxygen makes up only about one fifth of the air. Almost four-fifths is nitrogen, another gas. In addition, there are small amounts of other gases, such as the noble gas argon. Carbon dioxide is important: Plants live on it. They absorb it together with water and then convert it into glucose. Plants need this substance to grow. The excess is oxygen, which they release back into the air.
In addition, there is steam from water in the air. It rises to the top where it is colder. There, when the humidity has become very high, it becomes liquid water again, which returns to earth as rain, snow or fog. That is why there is hardly any water vapour in the entire atmosphere: less than half a percent.
Finally, there are not only gases in the air, but also dust. Some of this is man-made. Factories also emit dust through their chimneys and cars through their exhaust. This is why there is a lot of air pollution, especially in large cities.