For a simple example, there have been crops that were genetically modified by chemists, in a lab, to be able to survive exposure to Round-Up, aka glyphosate pesticides. Many people took a surface look at "Round-Up Ready" crops, and thought that scientists had somehow engineered a plant that would create toxins within the plant like glyphosate. This is of-course absurd, and would never be allowed--or even considered.

At the same time, there have been issues where small farmowners have benefit from unintended cross-pollination of Round-Up Ready crops with their own crops. Although it was (most likely)never intentional, Monsanto(the maker of Round-Up Ready crops) pursued farmers for pay, claiming that the farmer's owed Monsanto for the crops endowed with the Round-Up Ready genetic traits. This, of-course was an attempt at forcing farmers to buy only Monsanto products, instead of re-planting their own seeds, or buying seeds from other sources.

So, TLDR, and I am doing this all from memory, sorry guys, no sauce:
GMO products are actually not harmful to humans, they don't create unhealthy toxins, they are actually great for increasing the productivity of land and reducing the amount of soil amendments needed to maintain said land.

At the same time, Monsanto as a corporation, is very greedy, and wants to satisfy shareholders, so they've gone about using some very dirty methods to force farmers to use their products.

It's a mixed bag.