Atoms contain electrons. When an atom is not in an excited state, the electrons have the lowest possible energy. This is known as being in ground state. Electrons can have an infinite number of discrete energy levels. If one visulises theses levels as concentric circles around a nucleus they begin to converge, as every step between energy levels gets smaller.
The diagram is very basic as electrons can posess many more levels than shown.
The distance between each energy level corresponds to the amount of energy an electron needs to be given to 'jump' to it. Once an electron has been excited to an energy level is does not remain there unless continually being supplied with the required energy, instead it emits the energy is was given, and jumps back down to ground state. Electrons require a specific frequency of energy to excite up a level.
Visible light is made up of many different frequencies. If an atom absorbs absorbs its required certain frequency of energy from visible radiation (light) an electron is promoted from ground state to one of several possible levels; the atoms is in an excited state. It does not remain excited for long, as the energy absorbed is re-emitted as the frequency it absorbs, producing light of the colour related to the frequency.
It must be noted that when a molecule becomes excited, it can drop to an intermediate energy level by releasing a smaller packet (quanta) of energy and loosing the rest via vibrational energy in collisions. This vibrational energy is converted into kinetic energy and molecules in a sample will move more vigorously causing the sample to become warm.