Plain English Meaning
It's the buffer of sky around a route or procedure that air traffic control and procedure designers reserve for you, so that as long as you stay reasonably on course, you won't run into another aircraft or terrain.
Full Definition
The volume of airspace surrounding a navigation route, holding pattern, or procedure that is kept clear of conflicting traffic and obstacles to ensure safe separation when an aircraft is operating within established tolerances. Its dimensions are determined by the type of navigation system in use, the , and the expected of the aircraft.