This was a term I heard back in my religious days, describing a Christian who dated a non-Christian in the hopes that they could "save" the other person. These relationships rarely worked out very well, as you can imagine.
But the basic concept applies to any relationship. We can't "save" anyone, especially partners. All we can do is stand by them while they fix themselves.
There is a danger to this, though. People always rub off on each other, but it's easier to be influenced by bad behaviour and attitudes than by good behaviour and attitudes. In other words, if you have a toxic partner and a healthy partner, it's usually the toxic person that influences the other person the most.
Think of it this way. If you're standing on a chair and offer your hand to someone to help them up next to you, which is easier? Being pulled down off the chair, or pulling them up to where you are?
Getting people to change their toxic mindsets and actions is really, really hard. They have to want to change, and be ready to make the tough changes. It's much easier to be pulled down by someone who isn't good for us than to get them to not be toxic anymore.
I'm a caregiver. I want to help people. But over many years of watching dysfunctional relationships in a variety of social contexts, I've learned that unbalanced relationships with toxic people rarely end well. Healthy relationships require BOTH people to be healthy partners. Relationships aren't 50/50, but 100/100. One person cannot bear the weight of an entire relationship for very long; it will fall apart and the once healthy person may not be so healthy anymore. Both people have to give their all, and be heathy enough to successfully navigate relationship pitfalls.
And when we're talking poly this is even more crucial because now even more people are at risk from the toxic person's influence and actions. Poly is only as healthy as the group's weakest link. Choose your "links" very carefully.
(We don't have to be perfect, and of course we need to help each other grow and there will be moments when we need to carry the entire load for them when life stuff happens, but this is different than being paired with someone who is toxic.)
When someone is toxic, they need to fix themselves before they can be in a healthy relationship. If we get too close while they're still unhealthy for us, we will likely cut ourselves on their broken pieces and become a needless martyr. We may also unintentionally enable them to continue as they are, since they already "won" us and now have little motivation to change. We need to let them change on their own, and become capable of handling a healthy, mature relationship BEFORE we get closely involved with them.
Our job is to make ourselves as healthy and mature as we can as potential partners, and also to wisely choose who we get involved with so we do not hinder our own personal growth or that of our other poly partners and friends. Choose your partners wisely; they will always affect how effective your own personal growth is.