Voldemort leaves the Death Eaters, saying he needs to protect his snake.
Harry can "see" Voldemort's emotional reaction/thoughts in his mind. He now knows that Voldemort's snake, Nagini, is the last Horcrux.
He tells Ron what he saw.
Even though everyone has encouraged Harry to shut off this connection to Voldemort's mind up (as it has, er, caused some problems in the past), Ron now implores him to actually try to see inside Voldemort's mind this one time to see where he is.
Harry does it, and he sees Voldemort having a conversation with Lucius Malfoy. He asks Lucius to bring Snape to him.
Harry now knows where to find Voldemort, and so the kids rush off.
On their way out of the castle, they check out the mess around them. There are lots of dead people, and lots of fighting is still happening.
There's good news, though: Aberforth has joined the fight. Guess he wasn't as jaded and defeated as he claimed to be, huh?