Quantum theory describes how subatomic units or "quanta" exist or don't exist and interact or don't interact.

Quantum theory replaces "classical" scientific theories. Although there are obvious differences in explanations of how things work, the main differences are philosophical. Classical physics describes one world and it is real. It might be hard to understand but you can if you try.  

Quantum theory, especially the Copenhagen interpretation, says that there is no "real" world beneath the quantum - only descriptions and effects.

Quantum theory is one of the most successful theories ever. It's predictions always work and its equations are accurate to many places. But many of its practitioners  say  to know it is to realize that you don't.