Three short lines, and almost nothing happens on the surface. Someone is being taught, and the teacher is someone they already know. The word "again" matters most here. It suggests this isn't the first lesson, and perhaps not even the first time this particular lesson has come around.
The poem doesn't say who the teacher is, and that openness is part of its appeal. It could be a person, an old mentor or parent, but judging by the wording it could just as easily be a place, a book, a habit, or life itself. Whatever it is, the speaker seems to receive it with quiet gratitude rather than boredom or surprise.
The mood is calm and warm. The line break after "a" slows the poem down, like a small pause before the final line arrives. There is comfort in the familiar, but also a gentle humility: the speaker admits there is still more to learn, even from someone they thought they knew well.