Thousands of rivers return to the sea, which means that rivers finally flow into the sea, or the sea is made up of thousands of rivers.

The high road goes into the clouds, which means that the mountain road is endless and enters the clouds. The road ahead is long. "It's a long way to go in Xiu Yuan, so I'll go up and down."

The implication is that if you have an empty valley, you can't stop looking for it.