[Harry said] "You're a lot braver now that you're back on the ground and you've got your little friends with you," said Harry coolly. There was of course nothing at all little about Crabbe and Goyle, but as the High Table was full of teachers, neither of them could do more than crack their knuckles and scowl.
"I'd take you on anytime on my own," said Malfoy. "Tonight, if you want. Wizard's duel. Wands only - no contact. What's the matter? Never heard of a wizard's duel before, I suppose?"
"Of course he has," said Ron, wheeling around. "I'm his second, who's yours?"
Malfoy looked at Crabbe and Goyle, sizing them up.
"Crabbe," he said. "Midnight all right? We'll meet you in the trophy room; that's always unlocked."
When Malfoy had gone, Ron and Harry looked at each other.
"What is a wizard's duel?" said Harry. "And what do you mean, you're my second?"
"Well, a second's there to take over if you die," said Ron casually, getting started at last on his cold pie. (Harry Potter and the Sorcerer's Stone)
What do the highlighted parts respectively mean?