Library items are VM templates,

vApp

templates, or other VMware objects that can be contained in a content library. VMs and vApps have several files, such as log files, disk files, memory files, and snapshot files that are part of a single library item. You can create library items in a specific local library or remove items from a local library. You can also upload files to an item in a local library so that the libraries subscribed to it can download the files to their NFS or SMB server, or datastore.