Everyone calls her strong, and the speaker knows that label hides the truth. The poem is a short, quiet protest against being misread. Other people see someone who keeps going and assume she must be coping. The speaker knows she spends every day "on the verge of tears," holding herself together without ever being allowed to fall apart.
The mood is tense and sad, with a hint of frustration. The shift from "they" in the opening lines to "you" near the end feels like a turn toward the people who have failed to notice, as if the speaker is finally saying it to their faces: "You don't have a clue." The last line, "she's not fine," is blunt and plain, which makes it hit harder. The speaker may be describing someone else or may be hiding behind "she" to talk about herself. Either way, the poem suggests that calling someone strong can make it harder for her to ask for help.