We all tend to gravitate to people who are like us.  When it comes to the family of God, the Church, this can be detrimental.  Jesus taught that we should focus on inviting people to our parties who cannot return the favor; in other words, do something for someone who does not have the ability to pay us back so that our intentions are not for our own selfish gain but rather to give to those considered less than ourselves.  Riff raff is another term for garbage, and many who are low in society are thrown off as such.  Just as Jesus came to the earth as riff raff, born in a barn, and he focused a great majority of His ministry on people of the lowest standing in society, we are to also reach out to the riff raff in our world because when it comes to eternity in Heaven, we are all equal.