What are the plants in Trigun? I know that they generate energy and they're used like power plants but there's also a person or creature of some sort inside. What are the plants exactly and how is it that Vash can communicate with them?
In the Trigun Universe, Plants are constructions, built by humans, to hold and house powerful beings that can create energy, food, and even water. Not much else is revealed about the beings themselves in either the manga or anime that gives away where they came from, how humans learned to harness their power, or what they truly are.

As for why Vash can communicate with them:


   Both Vash and Knives are "products" of the plant. It is unknown if they were born by the plant on the ship, or how they came to be, as it is said that they were found near the plant itself. Vash and Knives both show many characteristics like the plant, and also many dissimilarities. For example, both show increased control over their powers, including being able to generate guns at will, and both have a far longer life-span than humans, while aging to the mid-30's look faster. They are also able to communicate with the plants, as Vash demonstrates on a few occasions. However, unlike plants, they do not need to be contained in a special environment to survive, and they are male, where as every plant form that can be seen is female.