Bat guano is another name for the droppings of bats. These droppings can be used as a very effective organic fertilizer, as they contain Nitrogen, Phosphorous and Potassium -- the primary nutrients found in all plant fertilizers that help plants grow.
The guano of bats has an NPK of 10-3-1. This fertlizer lingo means it contains 10% (N)itrogen, 3% (P)hosphorous and 1% Potassium (chemical symbol K). Nitrogen causes faster growth and greener leaves. Phosphorous helps develop roots and flowers. Potassium improves plant health.
Seabirds also produce guano that can also be used as a plant fertilizer. It tends to have a higher nitrogen content that that of bats. Overall, guano is fast-acting and (suprisingly) has little odour.
There are several ways you can apply guano to your plants:
Not all nurseries and gardening stores will have it in stock. You may need to call around or turn to the web. You get several different varieties with some having different NPK values from that mentioned above. Make an effort to buy guano from suppliers who follow a bat-friendly approach. See Environmental Impact below.
The mining of bats' guano from caves has an environmental impact. A number of invertibrates make use of the guano so the mining destroys their habitat. Bats living in the caves during mining get stressed and may drop their pups or die of starvation which is caused by stress.
I suggest looking for guano mined in a bat-friendly way. These mines operate when the bats are migrating with the intention of minimizing the impact on them.