Quantum mechanics is mathematical models used to describe and predict things at the atomic and subatomic level– protons, electrons, states, orbitals, etc. As with all models, if is not an exact representation of the the area and, in fact, explicitly deals in probability. However it has proven good at predicting things at the subatomic level. The worthiness of a model or theory is based on how well it can predict things.
Physicists have discovered that things at the atomic and subatomic level don’t work the same way as at the larger levels– the human and cosmic levels. The traditional Newtonian laws of physics don’t apply and nuclear physicists will tell you that your intuition won’t work at trying to understand the subatomic world.
One of the most famous paradoxes of nuclear physics is that light and particles have both the qualities of waves and particles, which, in normal thought, is oxymoronic. Another quirk of quantum mechanics is that subatomic particles move to different states instantaneously– there is no in between (speeding up or slowing own) between states. It’s on the order of a car going from 0 to 100 miles per hour instantaneously.
A limitation in quantum mechanics is that exact measuring of light and particles is impossible (see the previous post Heisenberg’s Uncertainty Principle). Measurements are ‘fuzzy’ and quantum mechanics only predict the probability of where and how fast is a subatomic particle . However, these predictions have proven to be accurate enough to be very useful and applied to make products and inventions including lasers and nuclear bombs.