A plant toxin is a poison made by a plant. They're generally defense chemicals that plants make to protect themselves. I think that much is obvious, so you're perhaps trying to get at "the dose makes the poison." Toxicity depends on dose, but again, since there are no essential nutrients that you can only get from plants, why eat them at all considering that they all contain various doses of these chemicals that we don't fully understand? Who defines what the safe dose of each alkaloid or terpene is, and on what basis? What about long term chronic exposure?
For your reference here is a list of plant toxins. The list is not complete, and it contains both false positives and false negatives, since we learn more about these chemicals every day. https://www.agroscope.admin.ch/en/toxic-plants-phytotoxin-tppt-database
Also as a side note, I'd pay "health influencers" no heed if I were you.