This app is provided by Yokohama City. The URL of Yokohama City is as follows.
https://www.city.yokohama.lg.jp/

This app is used in the "Yokohama City Electronic Application / Notification System".
By using this system, you can easily apply for many administrative procedures from the browser of your PC or smartphone.
In addition, this app allows you to access your Individual Number Card and digitally sign your application.

"My Number Card" is a personal card that can be obtained by a person who is legally resident in Japan.
Please refer to the following site.
https://www.cao.go.jp/bangouseido/card/index.html
https://www.kojinbango-card.go.jp/

To use this app, you need a model equipped with My Number Card and NFC Reader.
This is to access the IC chip of My Number Card.