Someone who's all-knowing is aware of everything. Most religions describe God as all-knowing.
The narrator of a book is often all-knowing, seeing and having insight into the actions and thoughts of every character. When you were little, you might have also thought of your parents as all-knowing, and religious believers usually believe their gods are all-knowing. All-knowing means "knowing all," from know and its Old English root, cnawan, "to know or perceive."