Guano is a rich source of nitrogen, phosphates and potassium, all of which are essential plant nutrients.
Unless sterilised, guano also contains microbes which can serve as a compost activator (encouraging decomposition), a fungicide and a nematocide (killing nematode worms, although often nematode are beneficial).
You can use guano in many ways. You can just sprinkle it onto the soil around plants as a top dressing. You can dig it into soil to prepare it. You can make guano tea to water plants with by leaving guano in a bucket of water overnight.
The amount to use depends on the plant and on the guano. If you buy guano, you should be able to find out its NPK concentrations from the package. If you get it some other way, you can test it with a kit, or just take your chances. Gardening books, and the care labels on bought plants, tend to advise you on how much fertiliser a particular plant needs.