. The explanation of "Knowing friends across the sea is like being close to each other across the world" means: As long as there are friends within the four seas, they are like close neighbors even if they are far apart. The image vividly expresses the author's inner open-mindedness and deep reluctance to leave his friends. The author's advice to friends who are about to part ways: Close friends have deep love and affection. Even if they are far away in the world, they seem to be close at hand. It not only shows the deep friendship between the author and his friends, but also reflects the author's broad mind. It washes away the sad sentiment of farewell in the poem, and comforts and inspires people with its positive, healthy feelings and broad-mindedness, so it has become a famous sentence through the ages.