Photo Courtesy: YorkShire Graduates
That's probably a character you've seen a lot while talking about China's Great Wall 万里长城 Wànlǐ chángchéng
Wànlǐ means 10,000 Miles.
So it is use to describe that it is immeasurably long!
"ten-thousand" (wàn) is figurative just like how it is said in Greek and English myriad which means "innumerable" or "immeasurable".