This is a tomato plant grafted onto a potato plant. Because both these plants are nightshades they are able to share a root system. By cutting the top plant off the root system of the potato, you can replace it with the top plant of the tomato. The tomato uses the root system of the potatoes as the potatoes use the top plant food from the tomato. This saves lots of space and is a big food producer.
Keep in mind this is not a new plant. No genetic modifications took place. No breeding took place. Under normal circumstances one would not be able to plant these things close to one another as nightshades kill each other in proximity.