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%/&deg;C.  Seems quite clear.

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