"– Onko tämä kasvi myrkyllinen? – On."
Translation:– Is this plant poisonous? – Yes, it is.
There is no yes, the affirmative of onko is on so is this, is answered by it is
Well, yes and no. The thing is that the Finnish answer On is not curt in any way. It's a perfectly normal answer, fit for any situation. The longer Kyllä, se on would simply sound weird. So what is being taught here is that the longer Yes, it is translates to just On in Finnish. They're equivalent. Just Yes is not equivalent to On in this context, because in English it sounds curt. So the important part to learn is that you can respond to questions like this in a much shorter way in Finnish than English, without being rude in any way.