When you import new entries from a supported reference format, or fetch entries directly from the Internet, they are first shown in this window for inspection. At this point none of the entries have been added to the database you already have open (if any).
The inspection window allows you to discard entries you don't want to keep, and to perform some simple operations like generating BibTeX keys for the entries, or adding them to groups. If you are importing into an existing database, it is often easier to perform these operations before they are mixed in between the entries of the existing database.