To be omniscient is to know everything. This often refers to a special power of God.
If you combine the Latin roots omnis (meaning "all") and scientia (meaning "knowledge"), you'll get omniscient, meaning "knowledge of all." It would be nice to be omniscient: then you would know absolutely everything in the world. Many religions have a god who is all-powerful and omniscient. This is how a god is supposed to know when you sinned, or what's going to happen in the future.