For every unit of energy the Earth receives from the sun it has to radiate the same amount back into space in order for the Earth to maintain it's temperature. We we radiate too little energy we heat up and if we radiate to much energy we cool off. When we radiate the same amount that we receive we remain in balance and our temperature holds steady.
Here is a diagram to help explain.
You can see in the top left corner a yellow 100%. This is the starting energy from the sun. The yellow numbers represent solar radiation and the red numbers are terrestrial radiation. The most important thing to take away from this is that if you add the numbers along the top (starting from the yellow number 8 to the red number 20) you will see they add up to 100. So of the 100 units of energy that comes from the sun, 100 units are reflected or emitted by the Earth.
Please send me a note if you find this confusing because it is very important to understand this is you want to understand things like Global Warming and the Greenhouse Effect.