Despatch riders were used by armies to send messages between headquarters and the fighting soldiers. They might be delivering reports, but sometimes they took urgent messages which were often secret. It was a very dangerous job. Travelling between the lines of fighting armies the despatch rider could easily be blown up by a mine, shot by gunfire, or captured by the enemy.
Despatch riders always used to ride on horseback. In the 20th century they usually rode motorcycles. They were used a lot in World War I and even in World War II, although by the end of World War II electronic communications and cryptography had improved so that despatch riders were rarer.