What does "serialized" mean in SQL

I am trying to do tape backups of a SQL database using Seagate Veritas and I am getting a failure in the SQL backup job that references that the "operations on a database must be serialized. Reissue the statement after the current backup, CHECKALLOC, or file manipulation operation is completed." How can I tell what operation, procedure, etc. is causing this problem and how do I correct it?