A simple definition of decoction would be a simmered herbal tea. Decoctions are often best for harder botanicals like roots or seeds that are not as readily infused by steeping in a teapot. A solid standard ratio of plant matter to water is 1oz herbs to 32 oz water. Bring the herbs to a low boil and simmer for around 15 minutes, remove from heat and let stand for another 10 min, strain and enjoy. Some decoctions prefer to be simmered with the lid on, depending on the aromatics of ingredients.
Decoctions have a couple other uses aside from being used as a healthful beverage. You can add decoctions to baths for soaking. Another application is as a compress. Apply a clean cloth, soaked in warm decoction solution to the affected area, and keep it in place until it cools.