Disable annoying voiceover: "Use the tab key to navigate..." [duplicate]
Being enabled only on the Login Screen you must disable it under System Preferences / Users & Groups. If the preferences are locked, click the lock icon in the lower-left corner of the page and type and administrator password to unlock them. Click Login Options and uncheck the Use VoiceOver at Login Window.
VoiceOver, along with other features of this kind, can be enabled, and hence disabled, under System Preferences / Universal Access.