Its the dried powdered form of a resin obtained from a variety of the fennel plant. As its name suggests, the aroma of the raw spice is practically fetid, extremely acrid. Its sold either as whole bricks which can be pinched off & used as needed (the texture of the block ranges from leathery to just plain rock hard), either by dissolving in water or microwaving it to a puffed up ball which can later be grated into a dish.
Its mostly used as a powdered spice though. As terrible as it smells raw, the minute you add (just a pinch, NEVER more) it to hot oil, the spice odor transforms into a delightlful onion/garlic aroma.
Its extensively used in Indian cuisine, particularly in regional religious dishes where onions & garlic are prohibited.

I've never heard of it but the question caught my atention because "asa fétida" in Portuguese means "stinky wing".I thought I'd check the "joke" and ended up learning something new...

It is a resin powder and has a *very* particular taste. Use only 1/8 of a teaspoon, if that, in most dishes and in most recipes you can leave it out completely. Raw it's rather unpleasant to smell, once cooked it's slightly garlicky.

It is also known as hing and is used in Ayurvedic medicine as a digestive.

It's a powder we use in Indian food. Gives a certain tang....somewhat lemony but not as acidic. when using, use a very light hand.