Each aircraft has enough power in the backup batteries to facilitate a safe landing. The batteries keeping navigation, ELUs (emergency lighting units), emergency pressure or oxygen systems running at altitude, and radio equipment operational. Larger aircraft have control surfaces that run on these backups as well.
The text describes how backup batteries power essential systems in an aircraft for a safe landing. These include navigation, lighting, oxygen/pressure systems, and radio communication.  Larger aircraft also utilize their backup power for control surfaces.