The Six Tastes of Ayurveda
The concept of the Six Tastes is a cornerstone of Ayurvedic nutrition...and one of the reasons Indian food is so damn good :)
According to Ayurveda, everything is made up of some combination of the five elements- this includes us, humans, and the foods that we eat. Specific combinations of elements create specific tastes in food, and depending on your individual makeup, you'll want to eat more of some tastes and less of others. Let's look at the tastes and their corresponding elements:
The Six Tastes
Sweet = Earth + Water
Sour = Earth + Fire
Salty = Water + Fire
Pungent = Fire + Air
Astringent = Air + Earth
Bitter = Air + Space (aka: Ether)
Each taste has an effect on the doshas, and the idea is to use the tastes within your diet to help balance your imbalances, especially any digestive imbalances that may be happening. Your dosha will tell you which combination of elements you are predominant in and therefore which combination of tastes you should include most in your diet. If you don't know what it is, you can find out what your dosha and your doshic imbalances are here.
Below are some examples of foods that fall into each taste category, and their effect on the doshas. This is written by either a + or a - next to the dosha (V for Vata, P for Pitta, and K for Kapha). So if it has V-P-K+, this means that the taste is balancing and good for Vata and Pitta, and aggravating (increasing) for Kapha.
Sweet Taste V-P-K+
Ripe sweet fruit such as bananas, mangos, watermelon, and really ripe pineapple (unripe fruit is more sour)
Grains like oats, wheat, rice, corn, barley, etc.
Sweet veggies that come from the earth like carrots, beets, sweet potatoes, yams, and winter squash varieties.
Dairy products such as milk, cream, butter, ghee
Sour Taste V-P+K+
Sour dairy products like yogurt, buttermilk, sour cream, cheese
Fermented foods and liquids
Salty Taste V-P+K+
Sea veggies and creatures
Bitter Taste V+P-K-
Eggplant, zucchini, and dark leafy greens like kale, mustard greens, and collards
Many herbs and spices like lemongrass, neem, turmeric, etc.
Grapefruit, pomello, olives, bitter melon
Coffee & leaf tea
Pungent Taste V+P+K-
Garlic, onions, chili peppers
Ginger, black pepper, cayenne pepper
Many other herbs and spices
Astringent Taste V+P+K-
Pomegranates, pears, cranberries
Broccoli, cauliflower, asparagus, turnips
In an Ayurvedic diet, all meals should ideally include all six tastes, but depending on what's happening in your body at the moment, you'll want your meals to include more of some tastes and maybe very little of others. Though the Shad Rasas are an important aspect of Ayurvedic eating, there are additional ways to eat for your dosha too.
This is just a basic outline of the Shad Rasas, but hopefully it shed a little light on the Ayurvedic principle of taste, and maybe that longstanding question of why in the world we love Indian food so much! Here's a great IFLScience article that also addresses the question :)