In the Manchu and Qing Dynasties, what kind of customs did people mean by "entering the customs"? Which country did the famous poet Byron come from?

The entrance of the Manchu and Qing Dynasties refers to the "Shanhai Pass", which is located at the east end of the Great Wall of the Ming Dynasty. The Great Wall of the Ming Dynasty stretches thousands of miles to prevent the barbarians from outside the pass, mainly to prevent the Mongols. It was guarded by Wu Sangui at that time, and once this pass was passed It is the North China Plain, driving straight into the capital.

Byron was a British Romantic poet in the early 19th century.