"God does not play dice with the universe" is one of Einstein's most famous and somewhat cryptic quotes.
His reference to God often causes people to assume he was religious — he wasn't. He was using God as a metaphor. In a letter he wrote in 1954, he said:
I do not believe in a personal God and I have never denied this but have expressed it clearly. If something is in me which can be called religious then it is the unbounded admiration for the structure of the world so far as our science can reveal it.
And when Einstein said "does not play dice," he was referring to the unpredictable nature of subatomic particles in quantum physics.
In short, the quote aims to outline how weird quantum mechanics is as a theory. You can read more on Tech Insider.