A public share link is a publicly available link to a Thread.
If a thread has a public share link, people who has access to the thread share the thread with anyone, even if they don't have a Threads account.
The person who has access to the link will be able to
Read the entire thread
Read any discussion on the thread
If they want to participate in the discussion, or get notified for it, they'll need to sign in or sign up for Threads to add themselves to the thread.
Public share links must be enabled by your organization administrator. You can enable, allow, or disable under the Security Settings in your organization settings.