Plain English Meaning
Everything a pilot can see, hear, or use that is either in the cockpit with them or out beyond the windows. Good pilots stay aware of both at the same time.
Full Definition
A phrase used in aviation instruction to describe the two domains of resources, information, and situational awareness a pilot must manage during flight: those found within the cockpit (instruments, avionics, checklists, charts, crew, passengers) and those found beyond it (visual cues out the window, weather, terrain, traffic, ATC communications, and -based services).