cooling food means those foods which have cooling properties. each and every food has cool or heat properties. hot property food increase heat in our body which can lead to high blood pressure. Cooling foods have properties to cool our body.
example;( Hot) spices, tea coffee garlic ginger
(cool) mint cucumber onion watermelon melon lime .
hope this help you to understand :)

thank you africh ,

I didn't know about this part " hot property food increase heat in our body which can lead to high blood pressure.".

I would guess that any low-calorie foods would be cooling, as calories are actually a measure of a potential for heat production.

Certainly, fats produce heat. That's why fats are needed in extremely cold conditions. But we all do need SOME fats in the diet, so I wouldn't cut them out entirely, just lower fat intake. You need the Omega fats, and others, so you could have some oily fish (if you eat fish) or Flax seeds if you prefer the vegan option. And vitamin E. Don't stop eating foods that contain vitamin E.

I'm not sure if I have much more to add to this thread, but I just wanted to note that traditional Chinese medicine (TCM) has a similar concept with food being hot/cold (there are also warm, cool and neutral foods).
From what I know, 'hot' foods aren't necessarily bad -- If a person often feels weak or cold, for example, eating warm or hot food would be good for them.
As an example, dong gui (a herb known to be 'heaty') can be used as a blood tonic. Another example is the durian fruit -- It is said to be 'heaty' and provides a lot of energy.

As for cooling stuff, one of my one of my favorites is chrysanthemum tea (mainly because I'm used to drinking it whenever I get a sore throat or some heat-related illness :P). Also, where I live, people would eat mangosteens to balance the 'heatiness' after eating durian fruits.

I don't know how exactly TCM or Ayurveda works, but it's interesting to know.

Wow, I use the word 'example' way too much lol

In Ayurveda, how a food will affect the doshas, is considered. This is called virya or potency. Food is either hot or cold. This is not temperature, but rather the heating or cooling effect the food will have on the doshas. Cooling foods such as cucumber, fennel or melons are generally good for Pitta, while heating foods such as peppers, onions, tomatoes and mustard are generally good for Vata and Kapha.