When an email is sent to multiple people, the phrase "undisclosed recipients" means that the email addresses of the recipients have been hidden. The method of hiding the recipients varies depending on the email client, and it is important to know how this can be done should it be required.
If an email is not sent to undisclosed recipients, then all of the email addresses of the recipients are visible. This may be considered unprofessional, particularly in instances when an email is sent to hundreds or thousands of people. Additionally, some parties may prefer to keep specific email addresses private and request to have their email address masked.