Most of the Earth's atmosphere is made up of nitrogen and oxygen, which do not have much effect in regulating the climate. Other gases that occur in trace amounts (<1% of the atmosphere) have a much bigger impact even though they occur in relatively small quantities. These are known as the greenhouse gases.
Energy (light) from the sun passes through the Earth's atmosphere and is not absorbed by the greenhouse gases (due to its short wavelength). The Earth absorbs this energy and radiates heat energy at a longer wavelength (infrared radiation) back into the atmosphere. The greenhouse gases absorb some of this energy and radiate much of it back towards the surface whilst the rest is radiated out to space. This plays an important role in keeping the Earth's surface warm and able to sustain life. Without this greenhouse effect the Earth would be much colder and life on this planet would be very different. This effect is called the greenhouse effect, because it acts a bit like a glass greenhouse that traps heat creating a warmer environment inside the greenhouse
Since the industrial revolution (about 150 years ago), human activities have led to emissions and an increase in the levels of greenhouse gases. An increase in these gases in the atmosphere means the atmosphere traps more heat and allows less heat to escape to space, leading to an increase in the average temperature of the Earth's surface.
Why is this important?
Rising temperatures may lead to changes in weather, sea levels and affect how land can be used. These changes are commonly referred to as climate change. Scientists predict even a small increase in the average temperature (about 1°C) could lead to increases in more extremes of weather including increased rainfall in some places, rising sea levels and more heat-waves and decreases in some such as cold spells. There could also be greater risk of drought in some countries in Africa and Asia and possibly more intense hurricanes.
Although it is already too late to stop climate change completely, making changes now could prevent it getting much worse.