The roe is salted and flavored with Japanese chili pepper (togarashi). Deep red in color. Also called Karashi-mentai(-ko).
[Karashi-mentaiko. Photo Wikipedia]
The normal Japanese name for the fish from which these eggs come, the Alaska pollock, is Sukotedara. So it is a type of tara, cod. It is 60 cm long and lives in the cold northern waters. It is popular in stews and one-pot dishes (nabemono).
Sukotedara is also called mentai, from the Korean myonte. The custom to preserve the eggs of this fish with salt and add chili pepper for spiciness, in fact first appears in the 17th-18th c. in Korea, where such kimchi-type preservation methods were common (while Japan does not originally know any spicy foods). In the 20th c., traders from Hakata in Kyushu started to import this product to Japan. After the war, it was produced locally and became quite popular nationwide and now is considered as a "traditional" specialty from Hakata. The spread nationwide was also helped by the opening of the Shinkansen line to Hakata/Fukuoka in 1975.
Mentaiko is eaten as it is as a side dish, although in that case it may also be lightly roasted. It is also used as the main ingredient in a popular type of chazuke. Finally, mentaiko is also a good accompaniment to sake.
Mentaiko is also called loosely tarako, as after all it are cod's eggs. But in fact tarako in the first place refers to the cod eggs alone, without the chili flavoring that is typical of mentaiko.
[Plain tarako. Photo Wikipedia]
Interesting use is made of mentaiko by using it as a spread on baguettes which are then toasted (mentaiko-furansu). Mentaiko can also be used to create an interesting type of spaghetti (mentaiko-supagetti); even more common in that case is the non-spicy type of tarako-supagetti.