Quantum mechanics? Just hearing the name, we get sick. It seems to be something complicated (and is), but here I will try to explain this theory in simple terms. However, this does not mean that you will go to understand quantum mechanics. As Niels Bohr said, "who says he understands quantum mechanics, you have not understood it." Does it look like something strange, doesn't it? Yes so. Quantum mechanics challenges our common sense.
Quantum mechanics is the theory that describes the behavior of the universe at the microscopic level; are composed of atoms and subatomic particles and quantum mechanics explains the behavior of these basic units. Hence, the name: Mechanical (explain behavior) quantum (the microscopic world, the quantum).
There are three explanations for this schizophrenic behavior of nature at the level of atoms and subatomic particles, but none of them are officially accepted by the scientific community. We can say that the most important is that quantum mechanics works, and explanations may be to philosophers.