Quantum physics is a branch of physics that works with the activities going on inside of atoms. They were wondering what was going on inside those things that were once thought to be solid. One big idea they came up with was that the energy of an electron depends on the frequency or wavelength. Scientists now say that electrons behave like waves, and fill areas of the atom like sound waves might fill a room. The electrons, then, exist in something scientists call "electron clouds". The size of the shells now relates to the size of the cloud.
Basically, quantum theory is the study of the tiny little atoms that make up everything, they do not behave like solid objects you see before you, they have their very own set of rules, and this is why they are so fascinating.