I had to edit your question as what you were asking for wasn't very clear. This poem does employ a very powerful idiom, but what you quoted in your original question certainly wasn't an idiom. An idiom is defined as an expression that is peculiar to a certain language and cannot be understood by a mere literal definition of its individual words. These are very difficult for foreign language learners to understand, as the phrases, such as "to have the upper hand," or "I played my ace," have no literal meaning if you understand the words themselves, but point towards comparing a situation with another one.
In "Miss Rosie," this tremendous poem lamenting the "destruction" wrought by age, the poem ends using a powerful idiom: "I stand up." This doesn't literally mean that the speaker does stand up, rather, this idiom suggests the way that that speaker mourns for Miss Rosie and wants to express her outrage, sadness and grief at what has happened to her. Standing up is a symbol of solidarity and protest, and this is what the idiom refers to in this poem.