Gain map
A hidden greyscale picture inside a photo telling HDR screens how much brighter each part may glow.
See it in Depth terrain. Try a sample map, then set Render mode to Height map: another greyscale picture that steers each spot.
In plain words
Some photos are made to look good on both normal and HDR screens. They carry a normal picture, plus a second, hidden greyscale picture. That hidden picture is the gain map. White areas mean this part may glow much brighter, and black means keep it as it is. Normal screens ignore it, while HDR screens use it to light the bright parts.