This is the 2706th most frequent Korean word.
쓰면
"If used" or "When writing/using."
In this sentence, "쓰면" means "if (you) use (it)", referring to the action of using medicine.
이 약을 쓰면 두통이 나아질 거예요.
If you use this medicine, your headache will get better.
Here, "쓰면" means "when (I) write", related to the act of writing letters.
편지를 쓰면 항상 마음이 가벼워져요.
When I write letters, I always feel lighter in my heart.
In this context, "쓰면" signifies "if (you) wear", associated with putting on a hat.