Radiative equilibrium determines Earth's core temperature: it says energy coming to the Earth from the Sun is balanced by the energy leaving Earth transmitted back to space. If incoming and outgoing energy are balanced, Earth's average temperature remains stable.
The Sun provides shortwave radiation (mostly visible light). Some of this is absorbed by Earth which warms. Earth then emits energy back to space as long-wave infrared radiation which cools Earth. If Earth had no atmosphere, its equilibrium temperature would be about -18 deg C (hint, remember this number for the Module 3 lab!) which would make life on Earth almost impossible. The key thing is that Earth’s atmosphere contains gases: carbon dioxide, methane and water vapor. These gases absorb some of the infrared radiation emitted by Earth's surface, re-radiate some of this energy back toward the Earth’s surface, and reduce the rate at which heat escapes back space.
These gases in the atmosphere prevent Earth from emitting as much energy as it should back to space. This in a nutshell is the greenhouse effect — the key that makes our planet a nice place to live. You will learn more about that soon.