thats a darn good question and I don't know why its been made harder to find out something important such as why a backup failed, all the info should be in the errorlog. I can only think that they mean any associated errors that might be in the errorlog (such as OS error 112) or they mean any log file you might have your backup job writing to (log file from a maintenance plan for example).

Otherwise there is in fact no such thing as a 'backup application log'