What is the name of the board/wall/plank that keeps table tennis balls from running too far away from the table? In this image, it is the purple and red board/wall/plank around the playing area that has "LIEBHERR" on it.
These are known as "barriers." For more information on barriers, refer here.
A preferred brand of table tennis barriers is Stiga:
Here is an example of Stiga barriers being used during an informal competition: