Thoughts and words actually come from the same place, from the heart. So as we look at today's proverb, we can see that out of the hearts of men come evil thoughts and plans. Knowing that the Bible teaches that the heart is deceitful above all things and is desperately wicked, we can know that such thoughts and plans reside in the breast of every man alive. The thoughts of man's heart were at one point so wicked that God decided to destroy the earth in the days of Noah. That is how much of an abomination these things are to Him.
We read next that pleasant words are pure. This might be a little hard to understand unless once again we take a biblical viewpoint. Words that come out of the mouth originate in the heart first. Jesus said that out of the abundance of the heart the mouth speaks. Thus when someone has pleasant words - they come from a pure heart. There are thoughts behind them that are equally pleasant as well.
Thoughts and words are very important to the Lord - and should be to us as well. Jesus made it clear in the Sermon on the Mount that there was far more to a man than just his actions. There were the things he thought and did in his heart. A man might state emphatically that he has never committed adultery, but if he looks at a woman with lust in his heart, Jesus says, he has already committed adultery with her in his heart. These things are vital for us to grasp because they take religion out of merely acting good - and puts it where it should be - that our hearts are transformed to be good at their core. This is wisdom that cannot be refuted.