Most commonly they're the defense chemicals employed by plants to discourage animals from eating them, like saponins or tannins. Sometimes they're the chemical

Most commonly they're the defense chemicals employed by plants to discourage animals from eating them, like saponins or tannins. Sometimes they're the chemicals sprayed on plants by humans to discourage non-human animals from eating them, or non-edible plants from competing for resources. It might just be a chemical vital to the organism that coincidentially interferes with digestion of essential nutrients for other organisms, like phytic acid or oxalic acid. They might even be like capsaicin, to discourage one type of animal from eating a fruit (mammals), while a more preferred animal is completely unaffected (birds). Cassava, better known in the US as the source material for tapioca, contains chemicals that can easily decompose into poisonous HCN. If you crush the cassava, as when biting it raw, the decomposition enzyme mixes with the chemicals and produces poison. If you leach and ferment the root in water, the toxins are removed. If you grind the root into paste and let the HCN escape into the atmosphere, the toxins are removed. Without those toxins, the food is safe[r] to eat. Food is made of a lot of different chemicals, and digestion encompasses a lot of different catalyzed chemical reactions. Some chemicals will slow down, speed up, prevent, or enable reactions between other chemicals. A lot of that just gets oversimplified to "toxin" because the information has to be understandable to people who don't have a doctorate-level education in biochemistry. Because of that oversimplification, scammers can exploit the intentional vagueness and ambiguity to turn fear into money. In that case, the "toxins" are just bullshit designed to scare you into buying stuff. The easiest way to distinguish between fact and FUD is by asking the claimant to name the toxin. If they can say "cyanogenic glucosides produce cyanide poisons on maceration that interfere with the oxygen-carrying capacity of the blood" it's probably a real thing. If they say "big agri-corporations spray them on all the food to keep us unhealthy, fat, and depressed" it's probably bullshit.