What does plain mean?

There is nothing special about describing people or things.

Plain is a common adjective, meaning that there is nothing special or striking about someone or something. Usually used to describe people or things that look ordinary or boring. This idiom comes from Zhu Guozhen's "Rushing to Xiao Zhuang". Volume 19. In the "Shenhuiji", the doctor said: Fang Cheng has it, it is plain and the more peaceful it is.