This is the 13423rd most frequent Russian word.
накипь
Scum or residue, typically formed on surfaces due to boiling water or other liquids.
In this sentence, 'накипь' refers to the limescale deposit that forms in kettles or water heaters due to hard water.
Накопившаяся накипь в чайнике требует чистки.
The accumulated scale in the kettle needs cleaning.
Here, 'накипь' is used metaphorically to describe emotional tension or unpleasantness in a discussion.
Эта накипь в нашей дискуссии должна быть убрана.
This bitterness in our discussion needs to be removed.
In this usage, 'накипь' refers to the foam or impurities that rise to the top of a boiling liquid, like soup or broth.