Was a strand of memory. Harry took it in the vial to the headmasters office and put it into the stone basin and dove in. It's how we find out that Snape was good and that Harry has to die.
Remember in HBP all the silvery strands of memory that Dumbledor and Harry dive into to learn more about Voldemort? Same concept here. Harry had to know at the very end that he was to die for it all to end so the only way for him to find out at this point is to have Snape, who is at death's door and barely able to talk, share his memory, or have Harry walk into the headmasters office and have the protrait of Dumbledor explain it.
Remember in HBP all the silvery strands of memory that Dumbledor and Harry dive into to learn more about Voldemort? Same concept here. Harry had to know at the very end that he was to die for it all to end so the only way for him to find out at this point is to have Snape, who is at death's door and barely able to talk, share his memory, or have Harry walk into the headmasters office and have the protrait of Dumbledor explain it.