This app can be used to learn finger spelling, which is a system for hand-expressing katakana in Japanese sign language.
If you want an ad-free experience, you can buy the premium version here: https://play.google.com/store/apps/details?id=com.yubimoji_premium

Learn yubimoji! The Japanese fingerspelling alphabet can be used to represent both hiragana and katakana. You can find a premium, ad-free version at the link above.