Communication
Most couples who come in communicate perfectly well with everyone else. What has broken down is specific to this relationship, and it is usually not about vocabulary.
It shows up as conversations that start about one thing and end somewhere unrelated. One person who goes quiet while the other pushes harder. Interpretations that arrive before the sentence is finished.
In session we slow the exchange down enough to see what each person is trying to say and what the other is hearing instead. Those are often different things.
The work involves learning to raise something before it has built up, and hearing a complaint without treating it as a verdict on the whole relationship.