A sign-up bonus refers to an offer that casino gives to new customers once they sign up on their site. Casinos use these bonuses to attract new customers and provide them with some funds to start betting. Sign-up bonuses are not withdraw-able and should be used within a timelimit, depending on the casino.
When mentioning Japan, people often think about sushi and samurai, but the Japanese are also famous for being hardworking and humble. Most forms of gambling are banned in Japan, but online mobile gambling is growing fast in the country and it's easy to find attractive bonus offers targeting Japanese residents.