Jako is boiled and dried baby sardines. Baby sardines are widely eaten in Japan, and depending on the degree of processing, the product is called differently.
The raw baby sardines are nama-shirasu, boiled ones are kama-age-shirasu, lightly dried ones after boiled are shirasu-boshi, and further dried ones are called jako.
I grew up in the area facing the Pacific Ocean, and had many fishing villages near by. We ate raw baby sardines, and freshly boiled soft baby sardines a lot.
In New York, I have to use much drier baby sardines than I used to. But drying process adds flavor to food, extra vitamin D and longer shelf life.
They are good to eat by themselves as calcium and protein rich snack. It’s also good to mix into hot cooked rice with a drop of soy sauce and butter, or mix in furikake.
Jako is difficult to find, but some Japanese grocery stores carry.