From the Wikipedia page
The Ancient Tea Route (simplified Chinese: 茶马古道; traditional Chinese: 茶馬古道) was a network of mule caravan paths winding through the mountains of Yunnan Province in Southwest China. It is also referred to as the Southern Silk Road and Tea and Horse Road. From around a thousand years ago, the Ancient Tea Route was a trade link from Yunnan, one of the first tea-producing regions: to India via Burma; to Tibet; and to central China via Sichuan Province. In addition to tea, the mule caravans carried salt. Both people and horses carried heavy loads, the tea porters sometimes carrying over 60-90kg, which was often more than their own body weight in tea.
It is believed that it was through this trading network that tea (typically tea bricks) first spread across China and Asia from its origins in Pu'er county, near Simao Prefecture in Yunnan.
The route earned the name Tea-Horse Road because of the common trade of Tibetan ponies for Chinese tea, a practice dating back at least to the Song dynasty, when the sturdy horses were important for China to fight warring nomads in the north.