Under Project SETTINGS, you have probably seen the Public/Private toggle button. Also, upon project creation you are asked about this.
Private projects are projects that are accessible only to project members. You still have the option to allow for other Taiga users or anonymous users (including search engines) to interact with the project if you allow for that using the External User role under SETTINGS > PERMISSIONS, but this is not enabled by default.
Public projects are projects that are forced to be at least viewable for non members and anonymous users (including search engines) through External user role. They also show up under Taiga’s DISCOVER page.
The configuration flexibility allows for some overlap. You decide what’s best for your project. Please note, Public Projects do not count for the paid plans on taiga.io, they are always free of charge regardless of team size or number of public projects.