There is a database that contains all of the updates and package data for the system (APT). If another application (such as dpkg) is currently accessing this database, you'll get that error. It is usually one of two things:
1. There is an update/updates in progress in the background (in which case trying again a little later should be ok)
or 2. Something crashed/was killed while updating, in which case a lockfile may still exist which says that the database is already in use. In this case, you'd need to delete the file /var/lib/dpkg/lock by hand.