This proverb highlights the idea that there are events or facts which are very hard to believe because they seem impossible or improbable. A chicken dead from a mosquito bite (which is very unusual) and a boar stuck in the mud (which is very rare) are exaggerated examples to illustrate this point. In a way, it says that one should not believe everything they hear without verifying its truth. An equivalent proverb in English could be ‘Seeing is believing.’