Quantum mechanics applies to subatomic particles movements and speeds. It states that we cannot exactly determine them both at the same time. It is one or the other. So while we determine a 100% Speed on a particle, his position can change in a range of value. And the other way around. That made Einstein unhappy about quantum mechanics. He stated that " God can't play dice ".
Quantum mechanics is a local theory, but if you consider extreme situation like black holes or the universe during its first minutes, you need to refer to the quantum mechanics because in that extreme environiment g. relativity and other classic mechanics theory do not work properly.