Overwrites files that already exist when uploading, instead of storing multiple copies with unique filenames.
By default WordPress doesn't overwrite an existing file if you upload a new one with the same name and directory. Instead, it appends a number to the end of the filename in order to make it unique, e.g., filename1.jpg.
That isn't always the desired behavior, so this plugin makes it so that any files uploaded will automatically overwrite existing files, rather than creating a second file with a unique name.
UPDATE: This plugin no longer requires modifying WordPress Core. You can install it and it'll work automatically, without any other changes.