When you first log in to the mobile app, you can be prompted to save your login to your device. This allows you to share the same login information on both the web and in the mobile app, and helps you manage multiple accounts if you need that.
If your login details have already been saved, you'll see a popup on the login screen that will allow you to use that information.
To save your password, make sure Save Login Details is enabled before you enter your login information!
If you're not prompted to save your password, you can set up iCloud keychain to keep your passwords updated across your devices.