What does stay in front of the knees mean?

When you have passed the guard, you are in front of the knees - side control, mount, north south. From dominant positions, the bottom player's major goal is to get their legs back in front of you to reguard. When you prioritize staying in front of the knees, you can more easily respond to their attempts. Put things in the way to block their knee from getting back in front of you.