Nam Pla is a thin brown fish sauce that is fundamental to Thai food. It's made by fermenting small whole fish (usually anchovies) in brine and drawing off the liquid, which is then bottled. It smells quite fishy and tastes very salty so use it sparingly as a flavouring and as a condiment (although cooking greatly reduces its fishiness and simply adds a richness and depth of flavour to dishes).
It's a staple in Asian cooking with slightly differing versions in each country. Nam pla is widely available in Asian markets and supermarkets.
However, I don't believe it's Kosher, so substitute a light soy sauce. That will do the trick.
Good Luck, The Kitchener Rebbe