This is a poem by Bai Juyi, the whole poem is:
I give you a way to eliminate doubts, without drilling turtles and wishing.
The test of jade requires three days of burning, and the eloquence requires seven years.
The Duke of Zhou was afraid of the day of rumors, but Wang Mang was humble and did not usurp the time.
Xiang Shi died at first, and who knows whether it will be true or false in his life.
The overall meaning is: Don’t look at people for a while, but look for a lifetime. Every good person will be misunderstood and framed, and every bad person will deceive people.
The main purpose is to teach people that "a long journey will tell you a horse's power, and time will tell you a person's heart".