It means that Oracle is rolling back all active transactions. It has disabled all current session from performin any further actions, it has also disabled any new connections to the database, but in order to shut down a database in consistent state, it must perform a rollback of all the transactions that have not yet been commited at the time when shutdown has begun. If those transaction were "huge" it might take a while before the rollback (and thus shutdown) completes.

If this takes too long or in the event that database for some reason realy "hangs", you still have the option to perform a SHUTDOWN ABORT.