Morra is a very popular slang term in Mexico for "girl" or "chick." You won't find this in a textbook, but you'll hear it all the time in modern music and casual conversation.
In "Ella Baila Sola," the song kicks off with the line, "¿Qué le parece esa morra?" (What do you think of that girl?), immediately setting a casual, conversational tone. Learning this word is a key to understanding the song's authentic, regional flavor.
This song is all about love at first sight at a party. The singer sees a girl dancing by herself, and he's immediately hooked. He tells his friend, "¿Qué le parece esa morra? La que anda bailando sola me gusta pa' mí" (What do you think of that girl? The one dancing alone, I like her for me). He thinks she's beautiful and knows it, with everyone watching her.
So, what does he do? He walks right up to her and makes a wildly confident promise: "Voy a conquistar tu familia, que en unos días vas a ser mía" (I'm going to win over your family, in a few days you'll be mine). It's a bold move! And it works. She tells him he's crazy, but she likes that he's different. He's honest that he isn't rich, saying "No soy un vato que tiene varo" (I'm not a guy who has money), but he promises to give her his whole heart. By the end, they're holding hands, his friend is in disbelief, and the feeling is mutual: "A ella le gusto y a mí me gusta" (She likes me and I like her).