In OotP, when Harry first arrives at the Blacks house, Sirius mentions to him that Lord Voldemort is after a very powerful weapon that he did not have the last time.
'What's he [Voldemort] after apart from followers?' Harry asked swiftly. He thought he saw Sirius and Lupin exchange the most fleeting of looks before Sirius answered.
'Stuff he can only get by stealth.'
When Harry continued to look puzzled, Sirius said, 'Like a weapon. Something he didn't have last time.'
I never quite understood what they meant by this weapon? Did they mean Harry? How would that make sense, how could Harry be used as a weapon by Voldemort?