quantum mechanics what does it mean?

Quantum mechanics is a fundamental branch of theoretical physics with wide applications in experimental physics that replaces classical mechanics and classical electromagnetism at the atomic and subatomic levels. Quantum mechanics is a more fundamental theory than Newtonian mechanics and classical electromagnetism, in the sense that it provides accurate and precise descriptions for many phenomena that these "classical" theories simply cannot explain on the atomic and subatomic level. Along with general relativity, quantum mechanics (actually relativistic quantum mechanics, called quantum electrodynamics) is one of the pillars of modern physics.

Einstein's theories led to quantum mechanics but he actually never accepted quantum mechanics. He complained that "God doesn't play dice". If you would like a really good layman's explanation of Einstein's theories I recommend the book "Relativity Visualized" by Lewis Carroll Epstein.