A smile can be misleading, and the speaker wants to correct that. The poem talks about a woman who keeps smiling even when someone is hurting her, and the speaker warns the reader not to take that smile as a sign that she is fine, or that she likes being treated badly. The last line makes the point plainly: there is only so much she can endure.
The speaker sounds protective and a little weary, as if they have watched this happen before. The poem speaks to "you," which makes it feel like a gentle but serious warning to the person doing the hurting. Its short, simple lines add to that effect. There is no anger or shouting, just a calm statement of fact. The mood is quiet and sad, but there is tension underneath it. By ending on the idea of a limit, the poem suggests that her patience will not last forever, and that the smile may not last either.