Composting is a process that breaks down organic material into its basic components and then uses those, combined with natural additives, to produce a material that improves soil.
In general, things that are man-made tend not to be compostable, but things that are natural or made from a combination of natural materials can. For example, plant material and food waste (including liquids) can be composted. Most yard waste, such as grass clippings, cut branches and old soils can be composted too.
In its simplest form, the recipe for compost is a mixture of green materials, brown materials, some water, and oxygen. Learn the fundamentals.