If there is one ingredient linking southeastern Asian cuisines, it is probably that fermented, smelly, salty, amber-colored substance known as fish sauce. Whether it is Vietnamese nuoc mam, Thai nam pla, Philippine patis, or the thicker Laotian padaek, fish sauce is key to Asian cooking. Consider that, in Vietnam alone, 95 percent of households use fish sauce regularly. I'm most familiar with Vietnamese cuisine, which I've spent much time trying to learn, and it's pretty difficult to think of a dish I've had recently that hasn't needed a glug or two of nuoc mam.
It's funny, my SO is Vietnamese, and he despises the stuff. I've asked him how he deals with the constant presence of fish sauce in the food he eats, and while he acknowledges the ubiquity of nuoc mam, he prefers to avoid watching the food preparation--and as long as his food isn't overly fishy or briny, he can pretend it's not there. Meanwhile, I can scarcely resist taking the occasional sip out of the bottle when I'm cooking (gross, I know--I think my BF and I fall on opposite extremes). And it's not only key in Vietnamese cooking. If you like that melody of sweet, spicy, and salty found in many Thai foods such as tom yum or tom kha gai, you have nam pla to thank.
Perhaps it's best to start with fish sauce basics. What is fish sauce? Essentially, fish fermented with salt. Appetizing, no? From the vats of fermented fish (often anchovies, and they're fermented for a year or more), the familiar amber-colored liquid is extracted and filtered several times before bottling. The result is a very salty, pure sauce useful in cooking, as a condiment, as a base for dipping sauces, and more. If you want to see this process in action, here is an entertaining video about the production of Thai nam pla:
And another depicting Vietnamese nuoc mam production in Vietnam's Quang Tri Province: