Answer:
Chinampa cultivation is a way that ancient Mesoamerican societies used to farm with. It's where you would have rectangular areas of land on the shallow part of a lake to grow your crops on.
The reason it was necessary on Tenochtitlan is because they were on a lake and the ground around the lake wasn't as good for farming as if they had just put the land in the lake. Because of this, they made Chinampas and grew their food on those. They also happened to be closer to the capital and made it just that slightest bit easier to go and get their food, too.
