Buddha's teaching is based on the four noble truths. The first of the four noble truths is the bitter truth. So, what is bitterness?
As soon as some people hear that Buddhism is bitter, they think that Buddhism is a bitter religion and a passive and pessimistic religion. In fact, the suffering mentioned here is not the pain and suffering that most people understand. What does that mean? Bitterness refers to the phenomenon of life here.
There is a very important Buddhist meditation work called Visuddhi-magga. Theoretical explanation: Bitterness, Bali is bitterness, which is composed of Du (lower)+Ha (empty). Because life is the refuge of many evils, that is to say, with life, there will be all kinds of imperfections, so it is inferior. At the same time, life basically does not have the essence of constancy, purity, happiness and self that those fools cling to, so it is empty. Because of inferiority and emptiness, it is called "bitterness".