BACKGROUND_DUMP_DEST specifies the pathname (directory or disc) where debugging trace files for the background processes (LGWR, DBWn, and so on) are written during Oracle operations.
An alert file in the directory specified by BACKGROUND_DUMP_DEST logs significant database events and messages. Anything that affects the database instance or global database is recorded here. The alert file is a normal text file. Its filename is operating system-dependent. For platforms that support multiple instances, it takes the form alert_sid.log, where sid is the system identifier. This file grows slowly, but without limit, so you might want to delete it periodically. You can delete the file even when the database is running.