Things that are done clearly can be easily understood. If you've clearly explained the rules before you start playing, your friends will have more fun and ask you fewer questions.
The adverb clearly is also a great word to use when something's obvious. If your little sister bursts into tears at the sight of the birthday clown, she's clearly terrified. And if everyone flubs their lines at the dress rehearsal of the school play, you clearly need extra practice before opening night. With your new glasses, you can see clearly, and you might say, "Clearly, I needed these!"