what is meant by "heat is an additive quantity"
Additive property ischaracterized or produced by addition, cumulative.
That is the definition for Additive property.
For example if a body has A amount of heat and another body has B amount of heat. The heat of both the bodies combined would be equal to A+B. That is the total heat is the sum of both.
This is not the case for temperature. Total temperature will not be equal to their sums.
That's the reason why it does not have an Additive property.