From Open Theism Simplified:

First off, as the name suggests, “Open Theism” is for “theists” …those who believe in a God. Secondly, “Open” describes possibilities. Put them together and you have possibilities (Open) and a God (Theism), therefore, “Open Theism.”

Ok, let’s go a little deeper into what is meant by “Open.” Possibilities are about what might and might not come to pass. If God is all-knowing, then it must also be the fact that God knows all of the future. Open Theists agree that God is all-knowing and that God knows all of the future, and like others, they think that some of that future is possibilities, but what sets Open Theism apart is the belief that God knows possibilities as possibilities. So, Open Theism then denies that God faces a future of only settled facts about what will or will not be, and insists that God knows the future as partly settled (or ‘closed’) and partly unsettled (or ‘open’). Only God knows all that is ‘closed’ or ‘open’ about the future.