Two lines are all it takes to describe a whole kind of relationship. The speaker listens to another person, but the words themselves aren't enough. What the person says and what they actually mean sit in different places, and the speaker spends their energy bridging the gap, like drawing lines between dots until a picture appears.
The feeling here is a mix of closeness and tiredness. To read someone this carefully, you have to care about them a great deal, or you've had to learn to out of necessity. Maybe the other person hides what they feel, or can't say it plainly. The poem doesn't tell us which, and that's part of its pull. The speaker sounds patient and perceptive, yet there's a quiet loneliness in always having to translate. The mood is soft, intimate and a little wistful, as if the speaker wishes they didn't have to decode anything at all.