Einstein wasn't the biggest cheerleader for quantum theory. In fact, he was always coming up with thought experiments to try to disprove it. But it was these thought experiments that challenged the pioneers of quantum theory to perfect it down to its finest details.
One of Einstein's thought experiments had to do with quantum entanglement, which Einstein liked to call "spooky action at a distance."
Imagine you have a two-sided coin that can easily be split in half. You flip the coin and, without looking, hand one side to your friend and keep the other side for yourself. Then your friend gets on a rocket ship and travels across the universe.
Now, look at your coin. You see that in your hand you're holding the heads side of the coin and instantaneously you know that your friend, who is billions of light years away from you at this point, is holding the tails side.
If you think of the sides of these coins as indeterminate, changing back and forth between heads and tails until the point in time when you observe one, then the coins can circumvent the speed of light, instantaneously affecting each other regardless of how many light years separate them.