Why does the stock_count method require a "="?

module Inventoryable 

  def stock_count
    @stock_count ||= 0 
  end

  def stock_count=(number) #????
    @stock_count = number
  end

  def in_stock?
    stock_count > 0
  end
end

class Shirt
  include Inventoryable
  attr_accessor :attributes

  def initialize(attributes)
    @attributes = attributes
  end
end

class Pant
  attr_accessor :attributes

  def initialize(attributes)
    @attributes = attributes
  end
end

class Accessory
  attr_accessor :attributes

  def initialize(attributes)
    @attributes = attributes
  end
end



shirt1 = Shirt.new(name: "MTF", size: "L")

shirt1.stock_count = 10
puts "Shirt 1 stock count: %s" % shirt1.stock_count
puts "Shirt 1 in stock?: %s" % shirt1.in_stock?