The answer to your question involves energy. Energy comes in a lot of forms, including light and heat. Energy can change forms. The energy from the sun comes to us as light, but it can become heat.

Light can be reflected or bounced back to where it came from. That's reflection and mirrors can do it.

Light can pass right through things like glass. That's called transmittance.

Light can be bent by things it passes through. Have you ever reached into a fish tank and noticed that things weren't where you expected them to be? That's because the light is being bent (refracted).

Light can be absorbed. If you put an object in front of a strong light,it will heat up. That's because the light energy has changed into heat energy. Heat is basically a form of light, but it has less energy than the light we can see. NASA has a site about heat at:

It may be a little hard to understand, but has great heat pictures.

Back to your question, getting through our atmosphere requires energy.Light, which has a lot of energy, so lots of it passes through our atmosphere. Then some of the light may be bounced back out (reflected). But some of it is absorbed by many things, such as the ground, water,trees, buildings, even you. It becomes heat. Heat has less energy than light, so it has a harder time getting back through the atmosphere.Some of it is trapped here.

This site has a nice picture of what happens to light as it enters and leaves our atmosphere:

If you don't believe that light can turn into heat, don't take my word for it. Have an adult help you with an experiment.

Fill 3 clear cups with water. Wrap one in foil, shiny side out. Wrap one in black paper.Leave one unwrapped or wrap it in clear plastic. Now shine a strong lamp on all three (it may take a while, depending on the light). Which water will get warmer? Hint: Which one is absorbing energy? Which one is throwing energy back where it came from? Which one is letting energy pass through it? Make sure you get help with this so that you don't get hurt by the hot lamp or start a fire.

Well it is not actually heat from the sun....instead it is LIGHT... the Earth's atmosphere is transparent to incident radiation (light) from the Sun. So the sunlight is able to easily travel through the Earth's atmosphere and strike the surface of the Earth... say the dirt. NOW, when this light strikes the dirt, the sunlight is absorbed by the atoms in the dirt. How does this happen? Well it happens by the following means: the atoms in the minerals making up the dirt are in constant vibration. Now when the photons from the sun hit the atom, the electrons in the atoms making up the dirt accept the energy and vibrate a little faster... we say that the dirt absorbs sunlight and heats up!!! So this is how the suns energy is picked up by the earth. Now it turns out that the dirt (or the atoms in the dirt) radiates off some energy themselves. But the wavelength of the OUTGOING energy is longer than the incoming energy from sunlight. And it turns out that carbon dioxide in the air absorbs some of the outgoing energy thereby trapping the SUN heat in the atmosphere.

So because carbon dioxide is transparent to sun light but NOT TRANSPARENT to the radiation given off by objects at the temperature of the earth surface, its a one way street...solar energy gets trapped by the earth preferentially in this manner.