The sign you mentioned is the Detour sign directing left. In Germany, this sign indicates that the main road is closed ahead due to roadworks or other obstacles, and drivers must take a detour to the left. It is crucial to follow this sign to avoid driving into an area that is obstructed or unsafe.

When you see this sign, you should prepare to change your route. Ensure that it is safe to turn left and follow the designated detour path. These detour signs are commonly used to reroute traffic safely and efficiently around a blockage, helping to maintain the flow of vehicles.

If you have any further questions about this sign or other traffic rules, feel free to ask!