The traboules are passageways that go from one building to another through courtyards and hallways. In other words, you can go through a doorway on rue St-Jean, go through a hallway, go up a flight of stairs, cross a courtyard and end up two blocks west without using any of the main roads. For many centuries, Lyon was a big producer of silk and in other to keep the cloths from getting wet during rainy days, the workmen used the traboules to get from the workshop to the merchants. The traboules were later used during the Second World War when the Nazis were occupying Lyon. In order to travel through city without them knowing, the people of Lyon would traboule (from the verb trabouler).
The stairways are mostly closed today as people live in these buildings and so many tourists could become noisy, but you can still go through the doorway and see the path the people of Lyon used in the past. Around 40 are still open and free to be used to the public but I only found one along rue St-Jean in the Vieux-Lyon.
Check out http://www.lyontraboules.net/ if you want to know more about the 230 traboules in Lyon or know more about the history (only in French though. Sorry 😦 ).