Güila is a colloquial term in Costa Rica, often used to refer to a young woman or 'chick'. It's a fun, informal word that adds a local flavor to the lyrics.
In the song, the artist uses güila to refer to other girls who might be interested in her partner, creating a sense of playful rivalry. It's an interesting word because it's specific to a region and gives insight into everyday Spanish slang.
Letty stamps her name on the beat and then spends two and a half minutes telling another girl exactly where she stands. The hook is the whole argument: "No se meta con mi mae" (Don't mess with my man), followed by the flat, funny escalation "Yo sí le doy con el bate" (I really will go at her with the bat). She is not actually worried, though. "Pero aquí no hay competencia" (But there's no competition here) lands like a shrug, and the boast underneath it is possessive and very specific: "Lo tengo pa' mí de lunes a domingo" (I have him to myself from Monday to Sunday).
The slang is Costa Rican and that is half the fun. A "mae" is a guy, a "güila" is a girl, and "El más chico de CR" (The youngest one in Costa Rica) plants the song in San José rather than San Juan. The last verse turns the camera around for a moment: she has plenty of admirers of her own, so "Pobrecito, mi mae" (Poor thing, my man) is teasing rather than kind, and she signs off telling him "que se quede tranquilo" (to stay calm).