From a spiritual POV, the idea behind karmic relationships is that the two of you made an agreement before your souls entered this lifetime, to help each other on your respective paths. Very often,there are numerous unresolved issues from past lives. When people open to learning rather than try to control each other, then much healing can occur.
In this way, karmic relationships are like guides or teachers. And usually, they're temporary. They're meant to help you grow at soul level in often difficult push-pull ways. But usually they are not intended to last a long time. This is because you are coming together in this life to clear karma between each other. It's often a balancing out of a bad experience from a past life.
But no matter what you call it, it does help to understand patterns in any relationship—especially toxic ones. Knowing if you're in a karmic relationship can help you move forward and work through whatever lesson is presenting itself through that person—so when you do move forward, you'll be ready for a love that lasts.