What is the meaning of "only begotten"?  How is the 2nd person of the Trinity, who is eternal, "begotten"?

Thanks.

This is part of revelation. It means that Jesus' relation to the first person of the blessed Trinity is that of a son to his father. How this comes about is a mystery.

There is a story about St Augustine. He wanted to understand the Holy Trinity. He was walking along the beach and he saw a child trying to pour water from the sea into a hole. He asked the child what he was doing. The child said that he was trying to pour the sea into the hole that he had just made. Augustine said that that would be impossible. The child replied, "It is also impossible for the mind of man to understand the infilinte mystery of the Holy Trinity." Then he disappeared.