Albert Einstein wrote: “Quantum theory yields much, but it hardly brings us close to the Old One’s secrets. I, in any case, am convinced He does not play dice with the universe.” It was addressed by Einstein to Max Born (one of the fathers of Quantum Mechanics) in a letter that he wrote to Born in 1926.
Einstein’s disagreement with the fundamental concept of Quantum Mechanics that at the quantum (i.e. atomic) level nature and the universe are totally random, namely events happen by mere chance. It uncertainty that Einstein disagreed with.
If quantum principles use to probability of throw dice, only predict how probable an event is to happen (like how many probabilities are there for six to come out when we throw dice). To express that he strongly disagreed, Einstein wrote, "He (God) does not play dice with the universe". Simply said, God as the creator, put an logical laws or principals in all creations, but science did not discover it yet.
Today, many people use the quote out of the context of quantum theory, or people used it in their own context. When you fall in in love with someone and becomes a lover, but few month then you found your lover cheated, and you said, "Why God send him to me?" I can said to you, "God does not play dice with the universe..."
For more explanation: https://www.stmarys.ac.uk/news/2014/09/physics-bey...