This is the 2434th most frequent Japanese word.
リラックス
"Relax."
Here, リラックスする is used as a verb, meaning 'to relax.' It emphasizes taking time to relax and unwind.
休日にリラックスする時間が大切です。
It is important to have relaxing time on holidays.
In this sentence, リラックスした is an adjective form describing a relaxed feeling.
この音楽を聴くと、とてもリラックスした気持ちになります。
When I listen to this music, I feel very relaxed.
Here, リラックス効果 is a noun phrase, denoting the 'relaxing effect' yoga provides.