I've received a couple asks about this so I want to talk about this! When we talk about effectiveness, we are referring to approaching a situation in the way which is most effective, and not necessarily what's fair or anything like that. It refers to stepping back from the situation to look at what works instead of focusing on being right.
This can apply in relationships, and general social interactions.
It would be so nice if things were always fair, but it isn't the world that we live in, and sometimes we need to adapt to get the best results we can or to prevent unwanted consequences.
Even if the other person is completely in the wrong, sometimes you need to step back and look at the consequences of the situation. Like if you're at your job, and dealing with an irate customer, screaming at them might feel validating but it could cost you your job. Should they be being rude to you? Of course not, but the fact is that we can't control other people's behaviour. We can only control our own. In this case, acting effectively would be doing what's needed to defuse the situation, and move them along. In some cases, you can do this by referring them to your manager but sometimes you are stuck making the painful decision of letting something go to placate the situation for your safety and/or well-being.
Another example of this is maybe you're driving down the road, and a car is trying to cut you off. You know they're in the wrong. You have the right of way and they're being obnoxious, but the safest course of action is to brake and let them in rather than risking an accident.
Sometimes it's relevant in situations with a loved one. Maybe you have conflicting needs like for example one of you is avoidant while the other one is the exact opposite. In situations like this, it's important to find compromises where ever possible. I had a friend who was really insecure when I wouldn't reply to her messages right away, and it was exhausting for me. We had disagreements over it. I felt like it was my right to have space when I needed it, and she felt like it wasn't fair to her because of her anxiety. Over time, we found a compromise. If I am too drained to respond to her messages, and it's carrying on longer than 24 hours, I send her an emoji that says "I still love you. I just can't talk right now." This has prevented disagreements and helps both of us foster a healthy relationship with each other.
A skill I find that helps with me with effectiveness is Radical Acceptance. In order to get through a situation, I learn to accept it. And it doesn't mean I am approving or saying the other person is right, but it helps me get through the situation to get the best results.
Please don't take this as you should always cave to people. That's not the point of the skill. It is important to have your boundaries, but part of the point of this skill is to help figure out whether it's the time for you to stand up for yourself by looking at the consequences of either action. Sometimes I allow smaller, less important things go to preserve a relationship but I dig in when something is threatening to compromise my morals or well-being. I weigh the consequences of making it my hill to die on with accepting it and letting it go. (The Pros & Cons skill can be helpful for me here!)