A gallery can have one of two privacy (or visibility) options - private or public. When a gallery is public, anyone with the URL for the gallery can view it. When a gallery is private, it means:
Only the client of the attached shoot can view it when logged in.
Anyone with the Access Key (optional) for the gallery can view it.
To change the visibility of a gallery, choose either "Private" or "Public" from the "Visibility" dropdown in the gallery settings window.
Once you've set a gallery to be private it will be locked down to only clients with access to it.
Creating a Access Key (Optional)
You will be able to optionally set a shared Access Key. The Access Key is a great way of allowing clients to access the gallery, without them having to create a username and password. The same Access Key can be used by anyone however, so be careful when you distribute it.