Effortless
Smooth
な adjective
Imagine something round that slides around with no trouble at all. Its movements are so effortless and smooth.
This is a jukugo word that uses the on'yomi readings of the kanji. You should be able to read this on your own.
英語で円滑なコミュニケーションを取るために、最も重要なことは何だと思いますか？
What do you think the most important things are to have a smooth conversation in English?