A scene library is a collection of scenes that have something in common.
A scene is the basic "building block" for creating a video. Each video includes one or more scenes which are configured in a storyboard. Learn here how to create storyboards.
New scenes can be created in After Effects using Idomoo's Scene Builder (plugin).
Scene Builder packages a video scene in the IDM file format which can be uploaded to a scene library.
Watch this video for a detailed explanation of how to create a scene library: