苦水 literally means 'bitter water' but is used figuratively to refer to deep-seated grievances, suffering, or complaints that someone has been holding inside — the kind of bitterness you 'spit out' when you finally confide in someone. You'll hear it in the common expression 倒苦水 (dào kǔ shuǐ, 'pour out one's grievances'). It can also refer literally to bitter-tasting water (e.g., from a well) or to bile rising to the mouth, but the figurative sense is the most common in everyday speech.