A long time ago, when I was making food for my second child, who was about eight months old, I made a soft puree of ripe avocado with cooked potato and a little olive oil. He loved it. I did, too—after I'd seasoned it with a dash of fish sauce for my adult palate. That led me to add fish sauce to guacamole the next time I made it. It turned out to be a great addition: Fish sauce enhances the avocado's richness and gives it extra depth without overpowering. Ever since, I've always added a dash to my guacamole—about a teaspoon. The effect is subtle (there's no taste of fish), but it's not a "hidden" ingredient: Taste the fish sauce version next to an unsauced one; the former is notably deeper, richer, and somehow smokier.
In many parts of mainland Southeast Asia, fish sauce (made from the liquid of, yes, salted fermented fish) is a staple, used for seasoning instead of salt. It gives wonderful depth of flavor—what the Japanese call umami. But unless you're cooking Southeast Asian food every week, a big bottle (it's cheapest in big bottles) can sit in your cupboard unloved and unused for months. It probably hasn't gotten into your regular rotation of cooking ingredients because, undiluted, it gives off an intense fishy smell.
Don't let the first whiff of fish sauce put you off; it's actually a lighter and more subtle seasoning than soy sauce. I prefer Thai fish sauces, though others like more pungent ones from Vietnam. In any case, it has become my most-used seasoning. I add it to soup broths, to vinaigrettes and marinades, and to many Asian dipping sauces (most often with lime juice and a little garlic), which are great for grilled meat, dumplings, or noodle bowls.