What exactly is 'American muscle'? In reference to cars.

I’ve always heard this phrase among my male friends, but I never asked what it was because at the time, I didn’t want to appear less manly haha.

So were the muscle cars from the past decades the best in the country or something? Or was there something special about them that put them in a higher echelon of car? As you can tell, I don’t know much about cars. Thanks.