I've heard a few people throw that term around when discussing navigation. What exactly does it mean?
Waay back before RNAV, GPS, integrated flight directors and autopilots, a primitive people used these means to navigate.
These indigenous people used basic attitude indicators in combination with a Direction Gyro, airspeed indicator, altimeter, and turn coordinator which were normally scattered about the panel in no particular order. To navigate they used airway markers, wearing a headset to hear a constant tone. As they would wander off course, they would hear dots or dashes indicating they were left or right of course.
As people evolved, a union called "ALPA" invented the "ALPA-T" or the "six pack", arranging the instruments in a logical, standard fashion based on rudimentary human factors training.
As time progressed, this became a standard for about 40 years. navigation went to DF (Direction finding - where you had to crank the antenna) to ADF (the antenna moved itself). Somewhere along the line, some inertial navigation equipment was invented (as the Germans weren't courteous enough to provide land based stations to aid our navigation). Then the VOR and ILS came along. You could track a course YOU selected off of a land based navigation system, and fly an approach that would end you looking at the runway. All with equipment you had in your cockpit, using ground based equipment maintained by the "Federal Government". We now know this as Slant Alpha.
Up until the mid-90s, about everything except the most advanced airplanes had no flight director, a six pack with a DG (unless you were lucky and had an HSI) and a wing leveling autopilot at best.
Then some computer nerd invented a way for this magical "glass" to integrate all the navigation features from boxes all over the panel that the pilot had to pick and choose which to use at the right time using a "scan" to get the pertinent information for that phase of flight.
As soon as glass became widespread and cheap, the devolution of the "raw data" flight set was complete, save some unsavvory characters known as "Freight Dawgs".
So now with our waning sense of history, occasionally pilots of glass filled flying machines will switch off the flight director (near emergency), the autopilot (Are you F'n spoofin' me?), switch the CDI from a GPS source to a ground based nav source and pretend they are over Ho Chi Minh city flying a C123 full of illicit arms to drop, while shooting a straight-in visual approach to ICT.
That my friend is raw data.