The error message means that a previous installation or update was aborted or
was left incomplete and the previous one needs to be either removed or
completed. You cannot leave it as it is or you won't be able yo update or
install. This should sort itself out, unless you have been installing things
outside of the repos which can leave you in a situation where you have to
manually remove the packages. Not a pretty thing.

Open the terminal and type: sudo dpkg --configure -a

You will be prompted for your password to sudo. Then it will tell you what needs
to be done, but basically just agree and type Y for Yes and press Enter.

It should take you back to a $ prompt when it is done. If this does not work
then let us know precise error messages that you see. Also make sure that when
you do this that you have no package manager open. No Synaptic or Update Manager
windows should be open. It is okay to have it in the system tray, but you cannot
use the dpkg command while a package manager is running.