The title explains it, but why should have to use it when I am installing a package via Terminal on my box then the installation fails, why is this and what does "dpkg --configure -a" do to fix it that the ordinary "sudo apt" cannot do?
From the manpage:
--configure package...|-a|--pending Reconfigure an unpacked package. If -a or --pending is given instead of package, all unpacked but unconfigured packages are configured. Configuring consists of the following steps: 1. Unpack the conffiles, and at the same time back up the old conffiles, so that they can be restored if something goes wrong. 2. Run postinst script, if provided by the package.
synaptic etc. are frontends to the same tool:
dpkg is very low-level, and
apt isn't particularly smart. So if package configuration was interrupted,
apt tells you to descend to the lower level to fix it. Note that
apt may not be aware of the packages which caused the problem (they may be manually installed local packages, and they may be a hundred packages and not one) and keeping it simple, it tells you to use the tool that is aware of them. Mitch has already posted what this command does.