The sign you are referring to is known as the "Advance warning arrow and lane boundary to signal the end of a traffic lane." This sign is used to alert drivers that a particular lane will soon be closed or will end. It indicates that they will need to merge into another lane safely.

When you encounter this sign, you should begin to prepare for changing lanes safely. Make sure to check your mirrors and blind spots, indicating your intention to change lanes to other road users, and merge smoothly. It is important to pay attention to the behavior of other drivers and to maintain a safe distance from vehicles as you merge.

In summary, this sign is a warning to prompt drivers to adapt their driving to the changing road conditions ahead. If you have any further questions about German traffic rules or other signs, feel free to ask!