Position mode is an easy-to-fly RC mode in which, when the sticks are released/centered, the vehicle will level and fly a straight line ground track in the current direction — compensating for wind and other forces.
The throttle determines airspeed (at 50% throttle the aircraft will hold its current altitude with a preset cruise speed). Pitch is used to ascend/descend. Roll, pitch and yaw are all angle-controlled (so it is impossible to roll over or loop the vehicle).
Position mode is the safest fixed-wing manual mode for new fliers.
The diagram below shows the mode behaviour visually (for a mode 2 transmitter).
Centered RC RPY sticks – gives level flight that follows a straight line ground track in the current direction against any wind.