Angostura bitters is a concentrated bitters, or botanically infused alcoholic mixture, made of water, 44.7% alcohol, herbs and spices, by House of Angostura in Trinidad and Tobago.
A common question is Why are Angostura bitters so damn popular? And this is a totally fair question!
Ultimately, the boring answer is -- they're historic, they're balanced, and they're really freakin' good. . (Plus, you have to love the packaging, complete with the iconic yellow cap.)
If you want to learn more about the history and production of Angostura bitters, Alcademics has a great writeup.
Plus, here's a great ad from 1952 for good measure: