Undertand the Collections in the Catalog panel in Lightroom
Some collections are created and maintained by Lightroom. You will find these in the Catalog panel in the Library module.
One of these, All Photographs, is a collection of all the photos in your Lightroom catalog. If you need to search all your images, click this before creating a search or filter.
The Previous Import Collection contains the images added to the catalog during the most recent import. As soon as new images are imported, the older images disappear from the Previous Import Collection.
Added by Previous Export is a collection of images you recently exported from Lightroom. When you did so, you selected the option to import the exported images back into Lightroom.
Quick Collection is a temporary collection that you can use to store images temporarily.