Amanita is the Spanish name for a mushroom belonging to the Amanita genus, which includes both edible and famously poisonous species. In the song, “Amanita de Albarracín” creates a mysterious, nature-inspired image, while Albarracín refers to a historic town in Spain.
This is a love song written like a walk through the Spanish countryside. Guitarricadelafuente opens with "No hago más que pensar" [All I do is think], and then sets the bar for the whole song: "Todo precisa un despertar" [Everything needs an awakening]. Nothing here is rushed. Things ripen, the way he says they do "Igual que espigas / Dispuestas para el pan" [Just like ears of wheat / ready for the bread].
Then the person he loves walks in, and the language turns almost holy. "De tu cuello manan los brillantes" [Diamonds flow from your neck] uses manar, a verb for water welling up from a spring, so the jewels seem to pour out of her rather than sit on her. He calls her "Amanita de Albarracín" [Amanita from Albarracín]. Amanita is a family of wild mushrooms (the red one with white spots is the famous one), and Albarracín is a real medieval village in the mountains of Teruel, in Aragón. Naming someone after a forest mushroom from a stone village is a strange compliment, and a lovely one: she is something rare that grows where few people look.
The end is plain and close: "Pecho a pecho, mira mi semblante" [Chest to chest, look at my face]. Semblante is an old-fashioned word for face or expression, a bit like saying "countenance" in English. And then the simplest line in the song, the one you can use tomorrow: "Qué bonito verte por aquí" [How lovely to see you around here].