Guano is bird or bat poop. If you park your car near the harbor, your windshield may end up covered in seagull guano. Ew.
You'll most often find the word guano used for the specific bird (or bat) manure that's used as a fertilizer in gardens. In fact, the term is so common that some fertilizers are called guano even when they don't contain actual bird poop. Guano, a Spanish word with Quechua roots, is rich in nitrogen, potassium, and other nutrients that help plants grow.