。
Good medicine tastes bitter.
Literal
Good-medicine [topic-は] mouth [at-に] bitter.
A famous proverb with classical structure. 良薬 ('good medicine, effective remedy'). 口に苦し ('bitter to the mouth') — 苦し is the classical predicative form of modern 苦い ('bitter'). The に marks the domain (the mouth). An ancient way of saying 'the truth hurts.'