What is library?
All the images added to Eagle will be placed in the "LIbrary". The library is fundamental for the images you saved and their respective metadata (i.e. tags, notes, comments). You can consider the library as an album, that different type of photos are to be placed into different types of albums respectively. Unlike the regular folder, the library is independent of each other and has their own folders, tags, images saved individually.
For example, every single change you made in the "Graphic Design" library won't be applied to another library, let's say, "Architectural Design".