What does it really mean when the scripture says, "And if your eye causes you to sin, pluck it out… It is better for you to enter into life with one eye, rather than having two eyes, to be cast into hell fire"?

A better understanding may come from the Amplified Bible:  "If your right eye serves as a trap to ensnare you or is an occasion for you to stumble and sin, cut it off and cast it from you.  It is better that you lose one of your members than your whole body be cast into hell (“Gehenna”- the Greek name for the garbage dump outside of Jerusalem. It symbolizes complete destruction – not eternal torment).” Matthew 5:29

If you have a depraved pleasure or habit that is as important to you as the use of your eye, and is contrary to God’s Law, it is better to part with that pleasure than to chance being alienated from God's mercy. 

For a more detailed discussion of hell, please go to the app. category “Hell, Death/Grave” and read the answer to the question: “If someone is sentenced to hell in the hereafter, what do you think will be the real pain or punishment that one will experience, since we then, will not have a physical body but will have a spiritual existence?”