If you are an administrator on the LightRocket system, you will notice a prominent green 'publish' button placed in various locations on the archive management page.
You can publish a file (or files) by selecting it and then clicking on the green publish button (at which point it will turn red...simply click again to unpublished a published file)
Publishing a file means it will become visible/searchable on the front end of your system which is the public facing online interface of your platform. If you have an open archive, published files can be retrieved and viewed by any visitor to your online archive.
But publishing a file doesn't mean it has to be 'public'. You can use LightRocket's 'set access' functionality to limit access to a published file (or files) thereby making it (them) visible only to those with corresponding authorisations.