This is the usual standard power de rating curve for silicon devices.
What it is saying is that the junction temperature must never exceed 150C.
The hotter the junction gets, the less additional power you can feed into it.
If your heat sink is already at 150C, then the rated power will be zero, because any extra power fed into the device would raise the junction temperature above the 150C maximum.
So the colder you can keep your heat sink, the more heat can flow out of the junction, and the more power you can feed into the device without the junction temperature exceeding 150C.
Devices are typically rated for a heat sink temperature of 25C which is just about impossible to achieve in practice, but it is the standard used.
A hypothetical device could have a rated power dissipation of 275 watts at 25C and de rated to 0 watts at 150C using the above curve.
A different way of rating the same device might be to say 275 watts de rated by 2.2 watts per degree Celsius of heat sink temperature rise.
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If the temperature rise of the heat sink (above 25C) is 125C and you multiply that 125C rise by 2.2, you must de rate the device by 275 watts.
As its only a 275 watt device, de rating by 275 watts gets you to zero at 150C