To describe is to report details about something either out loud or in writing. "Since we can't come to Paris with you, please describe it in your letter!"
Describe is from the Latin word describere which means "to write down." When you describe something, you're "da" scribe, the person writing things down! Writing teachers love to have students describe, using lots of details using the five senses and figurative language. It doesn't have to be written down, though — you can also describe that gold dress you saw to your friend on the phone. Or you can describe a circle, by sketching it out with a stick in the sand.