This sentence comes from Pipa Xing·Bing Preface (Bai Juyi).
The author was being demoted at the time. He lived in a remote place for a long time and lacked close friends, and he was depressed. When he met the Pipa girl who had been abandoned by a businessman, I learned that the other person had the same tragic experience, so I felt sorry for myself.
It means that we are all people who have fallen into the world. Now that we meet, no matter whether we have known each other before, we will feel sympathy for each other and respond with the same voice. So what? Why care about whether we have known each other before?