DC-DC Converter Datasheet - What does "Temperature Coefficient" Refer to?

Notice that this spec is in the "Output" section, so applies to the output:

It tells you how much the output voltage can change as a function of temperature alone. That spec is 0.03%/°C. Seems quite clear.

For example, if you change the temperature of the converter by 30 °C, then the output voltage can change by up to 0.9%. If you started at 12 V, for example, then after 30 °C change, you are only guaranteed to be within 12 V ±0.9% = 12 V ± 110 mV = 11.89 to 12.11 V.

The output measurements were taken at a certain operating and ambient condition (predefined load, ambient temperature, etc). The thing is that there is a variation in the output whenever there is a temperature increase or decrease. 0.03%/°C just means that the output varies within 0.03% whenever there is a 1°C change.

If the measurements were taken at 25°C and it is 30°C then you just need to do the math and you will get the expected output.