This sentence means that the person I love is separated from me by mountains and seas, which is a mountain and sea that I can never cross.

This sentence is extended from "The Complete Collection of Mountain and Wood Poems" by Li Bai, a famous poet of the Tang Dynasty. The complete sentence is "Love is separated by mountains and seas. Mountains and seas cannot be leveled. Mountains and seas can also be leveled. What is difficult to level is the human heart."

Translated as: The person I love is separated from me by thousands of mountains and rivers. The mountains and rivers cannot be filled up, and we cannot meet each other. Even if the mountains and seas can be filled up, the hardest thing to calm down is the human heart. It means that no amount of mountains and rivers can stop a change of heart. It is a hopeless love. Change "no" to "yes", that is, everything is possible and nothing can stop it.