This option is used in conjunction with the --unpack option to complete the installation of a package. This option may involve scripts specific to the package that put the finishing touches on the package, or it may require that dpkg manage the installation of files known as conffiles.
conffiles are files that have been declared by their package to require special handling during their installation. The passwd file, that holds the password information, is just such a file. dpkg provides the opportunity to replace the current password file with a new one whenever the package is updated. In the case of passwd this is usually not desirable, while with other packages, such as mimetypes, it may be perfectly appropriate. If you do replace an old conffile with a new one, the old file is backed up with the added extension .dpkg-old. If you reject the new conffile a copy is provided with the extension .dpkg-dist. This allows for recovery from mistakes, but it also provides the material from which a composite conffile can be constructed. Thus, with the password example, if major changes happened to the structure of the password file, the new file is accepted and the old data edited into the new file. This produces a composite password file with the new changes combined with the old user information.
This --configure option causes these remaining details of configuration to be completed for a given package. To configure all packages that are unpacked but not configured the -a | --pending option replaces the package name. To complete the installation of packages previously only unpacked, the command looks like:
dpkg --configure --pending
and will complete the installation of any unpacked packages.