expression, godan verb (-su)
boil water; get the bath ready
Used for heating water, especially for a bath. The second gloss is a common contextual extension.
お風呂に入る前に、まず湯を沸かしてください。
Before taking a bath, please boil the water first.
彼は毎朝、コーヒーを入れるために湯
Every morning, he boils water to make coffee.