Why are some of the words in the post highlighted?

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I have noticed that some of the words are highlighted, so I assumed they were associated with the tag lines, or the subject of the post. Now, that I am spending more time on this site, I see that is not true. Do the highlighted words dictate what you earn on your posts or responses? I hope someone can explain this to me. It does not matter either way, I will still respond to post that I find interesting and type what I feel.