Spray it onto a field and it kills everything growing there. But now Monsanto has genetically engineered a canola seed so that Roundup doesn't hurt it. That means a farmer can spray Roundup herbicide over an entire field, kill all the weeds growing there, and not hurt the canola crops, as long as it comes from Monsanto's special seed. But while farmers can buy the special seed, Monsanto keeps the rights to the DNA itself. That's what makes the seed special and that's where Monsanto makes its money.
Farmers traditionally plant their fields using seeds saved from their previous year's crop.
Just like in human beings, the DNA of seeds is passed along from generation to generation. So the problem for Monsanto is protecting its investment. In the brave new world of agriculture, it's Monsanto versus the farmer. Farmers buying Monsanto's seed must sign a contract promising to buy fresh seed every year. Then, they must let Monsanto inspect their fields for cheating.