Can someone explain what quantum mechanic is? in a nutshell

A branch of physics that deals with the structure and behavior of very small pieces of matter.

Since you have been able to locate this down to a nutshell, you really cant know anything else about it, not even what is it. ;)

In very simple terms, its the probabilistic way of explaining things that are even smaller than the smallest things we can possibly imagine to have a whole separate way of mechanics dedicated to explain their motion.

Its the principles that some times work, some times something else does and those seem like don't. :) But actually those principles always work.

Very small particles don't behave the way macro-things do and to formulate the bizarre, out-of-the-world ( but in the world ) way they do, scientists came up with a set of rules governed by some very hard and flexible rules that form the base of quantum mechanics.

Quantum mechanics is our formal mathematical way to understand how small particles move, behave.