What does it mean to read the letter when Lu died?

Lu's teacher, Elder Zhizhen, is a great sage in the water margin. Who can meet the future?

The four words he wrote to Lu are: catch in summer, catch in wax, listen to the tide, and be silent when you see the letter! In the evening, when Lu was resting in the Liuhe Temple in Hangzhou, he suddenly heard a letter from Qian Tang Bo and suddenly understood the meaning of the master's four sentences! Kill South General Xiahou Cheng and capture thief Fang La alive! Just listening to the tide letter, it should be dead!

I don't cultivate good fruits in my life, but I only love killing and setting fires. Open the golden rope abruptly and break the golden lock here. Hey! I didn't know who I was until the letter from Qiantang River came today.