Plain English Meaning
When you can see outside the cockpit clearly enough, you have to keep looking out the window for other aircraft and steer clear of them yourself. You can't rely on air traffic control or instruments to do it for you.
Full Definition
A regulatory concept requiring pilots, when weather conditions permit, to maintain vigilance to see and avoid other aircraft in flight, regardless of whether the operation is conducted under instrument flight rules or visual flight rules. The pilot is responsible for visually detecting conflicting traffic and taking action to prevent a collision.
Context Anchor
Used in traffic avoidance, visual flying, pattern operations, and any where pilots are expected to look outside for other aircraft.