I think compromise is important in any relationship, mono or poly. To me, it means that people start out on opposite sides of an issue and then each gives a little bit, and gives up a little bit, in order to meet in the middle.
I also think it's important not to hold onto grudges about what one gives up in compromises. Each party has to get on board with it all, as if it was their own idea and desire to give up what they did, in order to be happy. That also means don't give up something that will make you resentful for not having it. You have to embrace the compromise and own it fully in order to connect and move forward. We should not live in the past. Life is too short to hold onto resentments and relationships are not about keeping score. No one should be throwing it in anyone's face what they gave up and compromised on.