What does it mean that God is omniscience?

The word omniscience means all knowing.  This refers to the fact that God knows everything and that nothing happens without His knowledge of it.  God knows what has happened, what is happening, and what will happen throughout all the time and history of this earth (Isaiah 46:9-10).  Also, we cannot keep any secrets from God because He always knows what is going on all over creation (God knew that Sarah laughed when He told Abraham that she would bear a son; Genesis 18:13).  Since God is outside of time, He can look into it and see everything that has ever happened and will happen.  He also knows the outcomes of whatever decision we make or don’t make.  Just like God is present everywhere, God also knows all.