What is the Lightroom Library?
The Library module of Lightroom can be used to arrange, codify, systemize, group, and classify your media. When you import your images and video into the Library, you are adding information about your files to a catalog. Your files remain on your hard drive where they were, unless you are importing them from a camera, in which case the files are copied to your hard drive to a location of your choosing.
What is a Lightroom catalog?
The catalog is a database that stores information about the files you've imported into Lightroom. When importing a file, the first thing Lightroom does is collect the file's name and location on the hard drive. If the file has any attached compatible metadata, such as keyword tags, these are imported as well. You can use multiple catalogs and your catalogs can be carried forward from version to version.
Why does Lightroom say my files are missing?
Since the catalog database only stores information about your file, and not the file itself, if any of that information is changed outside of Lightroom, the connection to the file is lost. There are three ways this can happen: the file name is changed, the file location is changed, or the file has been deleted. If you need to rename, move, or delete a file, you can do so within the Library module, and then the connection is not lost.
What is Reconnecting?
When a file has been renamed, moved, or deleted, the catalog database needs to be updated with that new information. The Reconnection process allows you to point out a file's new name, it's new location, or to delete the database entry on your file if the file no longer exists on your hard drive.
Where can I learn more?
Managing Files in Lightroom's Library
Managing Catalogs in Lightroom
Reconnecting Media in Lightroom