A Confluence App is an application that implements additional features into a Confluence server. Apps are formerly know as add-ons or plugins.
The Universal Plugin Manager of Confluence allows to add additional features to a Confluence server. Typically apps are provided via the Atlassian Marketplace, but any software bundle that adhers to the anatomy of a plugin as specified by Atlassian, can be installed by a Confluence administrator.
Userscripts for Confluence is such an app.