Ni se diga más is a popular Spanish phrase that literally means "let nothing more be said". It's the perfect equivalent of the English expression "say no more", used when something is so obviously good or true that it needs no further explanation.
In the song, the singer is praising his love's beauty. After mentioning her ojitos (little eyes), he adds "ni se diga más", implying her eyes are so captivating that they leave him speechless. It's a cool, conversational phrase that perfectly captures a feeling of being overwhelmed by beauty.
This song is a super direct and sweet declaration of love. Emmanuel Cortes isn't playing games; he's telling someone exactly how he feels. He starts right off with "Mami, eres lo que quiero" (Mami, you're what I want) and explains that her smile is what "me debilita" (weakens me). He thinks she's perfect just the way she is, admiring her simplicity by singing, "Tan sencilla que tú eres / Para mí tú eres perfecta" (So simple that you are / For me you are perfect).
He's completely charmed by everything about her, from her "cuerpo hermoso" (beautiful body) to her eyes, which are so lovely he just says, "ni se diga más" (say no more). The whole song is his promise to show her all the love he has. He sees their potential relationship as a fairytale, calling it "un cuento" (a fairytale). It's a classic, heartfelt romantic song about being totally captivated by someone.