Tea horse trade road is well known to many tea drinkers, especially the Pu-erh tea drinkers. It's known as a trade route for tea and silk carried by horses. Well, it's partially true.
The true meaning is trading horses with tea, rather than silvers or copper coins. Tea was the designated currency when buying horses in the market. The early trade of horses with tea began in Tang Dynasty - 600's AD. At the time, it was not strictly regulated. By early Song Dynasty - 960's AD, horses trade between China and bordering countries/tribes were paid with copper coins which were later made into weapons. This act seriously threaten the security of the southwest and northwest borders, also the integrity of the Chinese currency. The Song central rulers soon banded the trade using coins. Textile, tea and herbal medicine were used for horse trading. New department for Tea Horse trade - Cha Ma Si were established along the borders, controlling trades and taxation of teas. According to document of such department duty, all horse trade among 4 bordering tribes, pay with tea only. 凡市马于四夷，率以茶易之.
Liao, Jin, Yuan Dynasties - 916-1368, Main land China was ruled by outsiders, hence horses were plentiful, therefore tea trades during the time were paid by silver/coins and local products. Beginning of Ming Dynasty - 1370, horse trading with tea was once again enforced by the central government. Such regulated practice continued through mid Qing Dynasty - turn of 1800's.