It is the shortest and most quoted of the Rimas: four lines in which she asks, her eyes fixed on his, what poetry is, and he hands the question back to her turned into an answer.
All of Bécquer's poetics fits in the last line. Poetry is not defined here: it is pointed at. «¿Qué es poesía? ¿Y tú me lo preguntas? Poesía... eres tú» — What is poetry? And you ask me? Poetry... is you. The answer identifies poetry with the beloved, and with that it settles the question without ever answering it.
The verse is kept in Spanish. Translating it in passing would replace the work with a paraphrase of it.