Kazunoko 数の子 is marinated herring roe, which is particularly associated with New Year cuisine, known as osechi-ryōri お節料理. Kazunoko literally means “number of children” and is symbolic of a felicitous wish for many descendants. The golden colour of the roe also symbolises prosperity.
Kazunoko is said to be an acquired taste: it is quite salty, but it is rich in umami flavour and has an interesting mouthfeel, whereby the little eggs separate and ‘pop’ in the mouth, giving the roe a complex, crunchy texture.
Preparation of the roe takes some time as the roe first has to be soaked in salt water, preferably several times. Although it seems counterintuitive to soak something salty in salt water, this process not only helps to reduce the briny taste of the roe, but also helps to remove the bitterness. Then the membrane surrounding the egg sac needs to be carefully removed. After this initial preparation, the roe is then marinated in a solution of dashi, light soy (usukuchi 薄口), mirin and sake.
You can find detailed instructions in English on how to prepare kazunoko at Just One Cookbook.