A OTP (One-time password) is a token that can be used to login into digital systems. In our case, it is a token that is sent via email. This way of logging in is useful for (external) users, as they do not need to create a password for their account.
As an admin, you can enable OTP workflows in the 'Admin Panel' under the 'Edit Instance' tab. The following options are shown there:
When one of these is allowed, the option to login with a OTP is displayed in the log in page as 'Access Token By Email':