These little fish is called Chirimen Jako (ちりめんじゃこ). These little fish are semi-dried salted tiny sardines and they are calcium-packed! They are often called Chirimen Jako or Shirasu Boshi (しらす干し). When they are boiled in salted water and not dried, they are called Kamaage Shirasu (釜揚げ しらす). Sometimes they are salty, so make sure to pour hot water before using.
You can find them in a Japanese grocery stores.
Recipes Using Chirimen Jako
I use chirimen jako as a topping for Cold Tofu (Hiyayakko).