A one-time password is a disposable password that is only valid once.
Authentication by email is a method of confirming your identity by entering a one-time password sent to the email address registered to your Expo ID.
Section "2. Types and Features of Identity Verification Methods" of the Expo ID registration manual contains detailed information on how to obtain an Expo ID, so please check it before registering.

ExpoID registration manual:
https://www.expo2025.or.jp/wp/wp-content/themes/expo2025orjp_2022/assets/pdf/tickets-index/id_registration_manual_en.pdf