我正在努力理解下面的代码。initialize方法中参数后面的冒号是什么意思？account：etc.我知道在变量名前有冒号意味着它是一个符号，不能像变量那样改变它的值。但是在变量名后有冒号意味着什么呢？谢谢

class Purchaser
  attr_accessor :consumable, :account, :amount, :reason
  def initialize(consumable:, account:, amount:, reason:)
    @consumable = consumable
    @account = account
    @amount = amount
    @reason = reason
  end

  def make_purchase
    if purchase.update(account: account, amount: amount, reason: reason) && decrease_stock
      return true
    else
      return false
  end