You can perform various tasks with the items in a content library. You can synchronize an item from a subscribed library to download all its contents and use the item to deploy a virtual machine for example. You can delete items you no longer need to use, and so on.
Each VM template, vApp template, or other type of file in a library is a library item. An item can contain a single file or multiple files. In the case of VM and vApp templates, each item contains multiple files. For example, because an OVF template is a set of files, when you upload an OVF template to the library, you actually upload all the files associated with the template (.ovf, .vmdk, and .mf), but in the vSphere Client you see listing only of the .ovf file in the content library.