Public Sims allow for access to Sims via a URL, without the user needing to sign in/register
For example, the following link will take you to a Sim that you don't have direct access to, but you are still able to view it.
Public Sims are great for quickly sharing a Sim to someone by just sending them a link, but this also comes with restrictions. Because the end user won't be logging in, you will not be able to track who has viewed your Sim or what results they achieved.
Public Sims can be activated in the Sim editor --> Properties --> Public Sims