Jaya,
An "off-host backup" is when you use an intermediary system to send the backup data to NetBackup.
For example: VMware backups via VCB are "off-host" because the VM files are copied from the VM host onto a 3rd party (the VCB proxy) and then sent to NetBackup from there.
Other examples of off-host backups might include things like: mounting a filesystem from a busy system onto a 3rd party and backing up from there. Or creating a FlashBackup with a SAN and then backing up that copy.
Hope that helps.