Morena is Álvaro de Luna completely lost to someone, and he is happy about it. He opens by saying he let himself get carried away and ended up in "una trampa mortal" (a deadly trap), wandering past the garden flowers until "me he perdido al entrar y no puedo salir" (I got lost going in and I can't get out). It is a love song that pretends to be a warning.
The chorus is all about her: "Tu piel tostada morena" (your tanned, brown skin) and "tu pelo suelto, mi negra" (your hair let down, my love). Heads up on that last one: "mi negra" is a very common affectionate nickname in Spanish-speaking countries, a bit like "babe" or "sweetheart", and "morena" means a dark-haired or tanned woman. Her hair "le baila al viento y se enreda con las estrellas" (dances in the wind and gets tangled with the stars), which is a lovely, slightly over-the-top image, and it works.
The bridge gets sweeter: "Solo tus caricias me hacen temblar" (only your touch makes me tremble) and "Robaré tu tiempo por si te vas" (I'll steal your time in case you leave). Great song for practising the perfect tense too: "me he dejado", "he llegado", "he jugado", "he ganado" all show up in the verses.