Hello my grandmother is watching this show called "Escaping Polygamy" with me now about a giant polygamist cult that has camps all over the us and it absolutely horrible what is happening to these women. Abuse, rape, incest, marriage to minors, etc. This show makes it seem that all of polygamy is abusive which is not true at all most relationships are understanding, healthy, and consensual. How do I explain this when I don't fully understand myself?
Polygamy as practiced by these dangerous cults is NOT the same as polyamory as practiced by consenting adults. I discuss the differences here. A lot of people find it easier to see the difference in things when there are different words for them. "Polygamy" is not the same as "polyamory," so you can start there. Polygamy carries connotations of cults, forced marriage, abuse, misogyny, etc. and polyamorous people do not use that word.
What you and your grandma saw in that show is real, and it's horrible. It is the truth of polygamous cults. Child marriage, patriarchal oppression of women, and sexual abuse are wrong in every way, and we should advocate against them. The show is correct that "all of polygamy is abusive" if we give over the term "polygamy" to those oppressive cults, which we've generally decided to do. Polyamorous people do not believe the same things, do the same things, or defend any of what the polygamous cults do.
Polyamory means "many loves," and it involves freely consenting adults living in love, joy, community, and freedom. It has nothing to do with religious or cult-based polygamy, and is in many ways the polar opposite!
It's sort of like saying "violence against your partner is always abuse and never okay" and also saying "consensual impact play in BDSM is healthy and fine." Both statements are true; they don't overlap, one does not need to yield or make excuses for the other. They are two completely different things, and it's important to be able to recognize the differences. We can honor the pain and reality of the victims of polygamous cults, and honor the freedom and health of consenting polyamorous adults, without having to hedge, make excuses, or compromise.
I'd encourage you to read through some of the introductory resources on my FAQ page, so you can have more examples, terms, and arguments for healthy polyamory next time it comes up. But don't feel like you have to equivocate or defend the disgusting practices of polygamous cults; it's not relevant in this case.