What does "Throwing Pearls to Swine" (Matt 7:6) mean?

So This is Christ delivering the 'Sermon on the Mount'. And I think he's talking about how to deal with other people because In the same passage, Christ talks about Taking the beam from your own eye, before removing the mote from your brother's eye. The verse directly after: >Do not give dogs what is holy; do not throw your pearls before swine. If you do, they may trample them under their feet, and then turn and tear you to pieces. (Matt 7:6) It seems to say don't preach the Gospel (what is holy, and was is -precious as- pearls) to the Dogs and Swine, which I can only imagine as Sinners. But I don't think that is correct! Because the Lord preached to Sinners, and even said that he came for sinners - and not the righteous. (Luke 5:32) So is the (Holy things/Pearls) not the Gospel? Who are the Dogs/Swine? And How do I know who is the Dog/Swine?