1.punch line
He tried to be funny, but his punch line made nobody laugh.
2.make out
It's hard for me to make out the letter's handwriting, neither for the writer himself.
summary:
Harry's uncle was angry with Dobby's noise, thinking that was made by Harry himself, and warn Harry not make any sound again. Dobby told Harry that he had hidden the letters to Harry from Hermione, Ron and Hagrid for stopping Harry go back to school. Harry didn't agree with Dobby and want to get the letters back. That made Dobby try to do something bad to scare the Dursleys. Harry was frightened.