Have you ever stepped into a car that's been sitting in the sun and you feel like you have walked into an oven. That is the "greenhouse effect."
What comes to us from the sun is light, not heat (except for some colors infra-red) that we cannot see. of wavelengths or colors (short wavelengths are blue, long are red) including all visible colors plus ultra-violet (short) and infra-red (long wave-lenghts). The infra-red is felt as heat, but not the other colors. One of two things can happen. If the sunlight strikes something that reflects the colors of visible light (white is best), it is reflected back into space and it has no effect on the earth other than to make it possible to see it from a space station. If it hits something dark (especially black) it is not reflected, which is why it looks black. The black has absorbed the light. The light is enery no matter what color it is, and one of the most fundamental laws of physics comes into play. Energy is "conserved." You can't destroy energy so light energy becomes heat energy and what has absorbed the light becomes warmer. The next quesion is where does the heat go.In the car, it warms the car and the inside of the car. On the earth it warms the atmosphere.
Now comes the greenhouse part. The heat from the sunlight doesn't all go into space. Different gasses in the atmosphere reflect some of the heat back to the earth. The gas that reflects the most heat is carbon dioxide. CO2 helps keep the earth warm enough for life.
That's the good part and before human beings came along everything was in balance, and on the whole the earth was staying at about the right temperature for higher forms of life.Now come humans and start burnng things primarily for warmth and cooking which doesn't add all that much CO2. However in the mid-1800s, steam engines come along, then home heating systems and then cars and the amount of CO2 in the air keeps getting greater. Right now we are adding 42 BILLION tons per year. This extra carbon dioxide reflects more heat back towards the earth. And that is THE problem. Except for one other. Warming the earth has various ways of generating more greenhouse gases, especially heating and melting the tundra, which realeases methane which is about 30 times more effectivve at reflecting heat as carbon dioxide. Now we have a real problem. Since the mid-1800s the earth and its atmosphere have been getting warmer and warmer.