Thermal resistance Junction - Maximum Power dissipation
Yes you should take that Thermal Resistance as a constant, it is a property of how a chip or device is packaged (how well the package can transfer heat) and if that heat transfer is "helped" in any way for example by using a heatsink.

If your ambient temperature is 50°C and you allow the junction in your device to reach for example 90°C then that means there's a 90°C - 50°C = 40°C temperature difference.

For a thermal resistance of 250°C/W that means 40°C / 250°C * 1 Watt = 0.16 Watt can be dissipated. Dissipate more and the junction in your device will become hotter than 90°C (assuming the ambient temperature is 50°C).