To be quiet means you don't make any noise. When you’re trying not to wake a sleeping lion, you will be quiet when you run by it.
The versatile quiet, which can be a noun, adjective, or verb (to quiet others), is one of those words that is best understood through its antonyms — loud or hectic. Let a marching band practice in your apartment, and you will long for a little quiet. Quiet is one of those words that thankfully follow the "i" before "e," except after "c" spelling rule.