The atmosphere is the Earth's air envelope. It surrounds the planet like a thin skin. It consists of air, which is a mixture of gases. The largest proportion is nitrogen. In second place is oxygen, which all animals and we humans need to breathe. Among other things, plants need the gas carbon dioxide to live, which they also get from the atmosphere. Without the atmosphere, there could be no life on Earth.
The air in the atmosphere is attracted by the Earth's gravity. This gives the air a weight with which it presses on the earth. We feel this weight as air pressure. It is not the same in all places on earth; there are areas with higher air pressure and others with lower. Air always flows from high to low pressure. We call these air currents wind.
The atmosphere also contains water. It can form clouds from which rain or snow sometimes falls to earth. So the atmosphere is where the weather is made.
Some other planets in our solar system also have an atmosphere, some do not. The atmosphere of another planet can contain completely different gases than the atmosphere of the Earth.