I don't understand hashes, RUBY

This code is in my book, there are 2 lines that I don't understand, I neel help

library.rb

class Library
   def initialize
       @books ={}  #creating a new hash
   end
   def addBook(book) 
       @books[book.category]||=[]          #category is a symbol. I DONT UNDERSTAND THIS LINE
       @books{book.category] << book       #AND THIS ONE
    end
end