When we think of Japanese soy sauce, we generally think of koikuchi—the Japanese word for dark soy sauce. This is the classic soy sauce used for dipping sushi or gyoza dumplings. It's great in stir fries, marinades, and sauces. If a Japanese recipe calls for soy sauce without any extra detail, it almost certainly means koikuchi.
This particular dark soy sauce comes from Yugeta, a fourth-generation Japanese company. The sauce is aged for about a year in traditional cedar barrels called kioke. Generations ago, that's how all soy sauce was made in Japan. Today, it's very rare: only 15 Japanese soy sauce makers still use kioke. The barrels are often in use for decades or centuries, much like the barrels that age traditional balsamic vinegar. Over the years, the barrels age batch after batch of soy sauce. They're used not to give a woodsy flavor to the sauce, but to help provide a consistent flavor to each new batch of sauce.
The dark soy sauce from Yugeta has a subtle wine-y, fruity sweetness that helps balance the salt. It's an ideal candidate for your every day soy sauce for Japanese cooking.
Finally, a note about recipe substitutions. If a Chinese recipe calls for light soy sauce, koikuchi is a good replacement. Note, though, that it's not a great option for Chinese recipes calling for dark soy sauce. In China, dark soy sauce is typically sweetened, and there's no good Japanese equivalent.