Plain English Meaning
It's the set of small air tubes that carry two kinds of outside air pressure into the cockpit: the pressure caused by the airplane moving forward, and the pressure of the still air around it. The flight instruments use those two pressures to figure out how fast and how high the airplane is flying.
Full Definition
The network of air-pressure tubing, ports, and fittings that carries ram (pitot) air pressure and ambient (static) air pressure from sensing ports on the outside of the aircraft to flight instruments or to an Air Data Computer. The pitot side measures the pressure of air being forced into a forward-facing tube as the aircraft moves through it; the static side measures the undisturbed atmospheric pressure surrounding the aircraft. Together these two pressure sources feed the airspeed indicator, altimeter, vertical speed indicator, and modern air computers.