It all starts with a farmer named Roscoe Filburn, a modest farmer who grew wheat in his own back yard in order to feed his chickens.
One day, a U.S. government official showed up at his farm. Noting that Filburn was growing a lot of wheat, this government official determined that Filburn was growing too much wheat and ordered Filburn to destroy his wheat crops and pay a large fine to the federal government.
The year was 1940, you see. And through a highly protectionist policy, the federal government had decided to artificially drive up the prices of wheat by limiting the amount of wheat that could be grown on any given acre. This is all part of Big Government's "infinite wisdom" of trying to somehow improve prosperity by destroying food and impairing economic productivity. (Be wary any time the government says it's going to "solve problems" for you.)
The federal government, of course, claims authority over all commerce (even when such claims are blatantly in violation of the limitations placed upon government by the Constitution). But Roscoe Filburn wasn't selling his wheat to anyone. Thus, he was not engaged in interstate commerce. He wasn't growing wheat as something to use for commerce at all, in fact. He was simply growing wheat in his back yard and feeding it to his chickens. That's not commerce. That's just growing your own food.
If you can control the food supply, you can control the people. It's like NAIS, only with plants.