What does Matthew mean by "if your eye causes you to sin, tear it out", etc.? What does this verse tell us? It's quite hard to understand.

Matthew 18:9:


  “And if your eye causes you to sin, tear it out and throw it away. It
  is better for you to enter life with one eye than with two eyes to be
  thrown into the hell of fire.”
Sin is not something we as Christians should fool around with. The same applies to non-Christians as well, for that matter.  Consequently, we need to get tough with sin and tough with ourselves when it comes to nipping it in the bud. 

Of course, a man could avoid lusting after a woman by blinding himself in both eyes, but such an extreme measure, interestingly enough, would not cure lust! I suggest that is why Jesus talked about plucking out one eye, not two.  

It's pretty clear, to me at any rate, Jesus is using hyperbole, which is a figure of speech which exaggerates deliberately for best effect. If Jesus had said,


  "If you are aware of what is causing you to sin, you need to have a little conversation with yourself in which you rebuke yourself and promise God you'll not sin again,"


His words would not have had the impact as would His words about eye-gouging. He knew that, of course, and He knew His words would cause His listeners to stop and analyze what they'd just heard, and in doing so they would've gotten the message.  In essence, Jesus was saying 


  "Get tough on yourself. Make a clean break with sin and with anything that causes you to sin. What you thereby 'lose' for my kingdom's sake will be rewarded richly in the kingdom to come. If you refuse to make a clean break with sin, you may just find yourself in hell one day because of the very thing you refused to give up!"


Very sobering words, indeed, for believers and unbelievers alike. Sinful, earthly pleasures are but for a season, but at God's right hand in heaven, there are joy and pleasure forevermore (Psalm 16:11). In other words, a little pain now will yield eternal rewards. Our rejection of pain now will yield eternal regret in hell. The choice is ours to make.

Allow me to alter Jim Elliot's famous words ever so slightly:


  "He is no fool who gives [up] what he cannot keep to gain what he cannot lose."


Jim Elliot, a missionary who sacrificed his life attempting to bring the gospel to a murderous tribe in the Amazon Rain Forest, was speared to death by the very people he came to serve (the Aucas).  He was an eye gouger.