open
Explanation
In this comic, a person in what appears to be a relationship or social setting says "I'm sorry, it's just not working for me. We're not open." Another person responds with confusion, asking if they mean "open" as in an open relationship. The first person clarifies that no, they want "openness" -- they want to have honest conversations about feelings and the direction of their relationship. The other person, surprised, asks: "How long weren't you open with me? We were together for..." The first person then says something like "We are going in the same direction" -- suggesting they were never really being open or honest with each other.
The joke hinges on the ambiguity of the word "open." In modern dating culture, "open" most commonly refers to an "open relationship" (non-monogamy), and that is where the second person's mind immediately goes. But the first person means "open" in the older, more fundamental sense: emotional openness, honesty, and vulnerability. The comic plays on how contemporary relationship vocabulary has been so colonized by specific terms of art that a basic English word like "open" now triggers a very particular interpretation.
There is also a layer of irony in the fact that the couple apparently lacks the very openness needed to even have a clear conversation about openness. Their miscommunication about what "open" means is itself evidence of their communication problems. SMBC frequently mines humor from linguistic ambiguity and the gap between what words technically mean and how they are culturally understood.