The speaker sits in an audience watching a woman who once starred in a musical he wrote in college. Thirty years have passed, and she has become a better actress. He notices that her body has changed, but her smile and her shy dignity haven't. He suggests those qualities were never an act, which is why they still feel so familiar.
The mood is warm, calm, and quietly proud. There's no longing or regret here. The speaker leaves the theater with a "hidden smile," pleased that both of them have lived full lives. The phrase "second act" cleverly describes middle age, and the final lines widen the idea: everyday life becomes a kind of performance, and both people have played their parts well. What the poem celebrates is the lasting memory of youthful creative excitement, "finding our light," and the contentment of seeing someone else carry it forward.