Parent rock refers to the original rock from which something else was formed. It is mainly used in the context of soil formation where the parent rock will have a large influence on the nature of the resulting soil. The term is also used in the context of metamorphic rocks where again the parent rock refers to the original rock before metamorphism takes place. Parent Rocks can be sedimentary, igneous or metamorphic.(ex.- shale becomes slate, granite becomes gneiss etc.) In these cases, parent rock may be referred to as the protolith

Protolith

Protolith refers to the precursor lithology of a metamorphic rock.For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other rock and thus have a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology.Sedimentary...

. Parent rock is the main source of soil. This type of soil is also called residual soil.
Different parent rocks have different chemical compositions. The Parent Rock is also known as bedrock and is made mostly of solid rocks, there is no weathering occurred there because the roots of the plant can not go that deep. It is the third layer and is underneath Topsoil

Topsoil

Topsoil is the upper, outermost layer of soil, usually the top to . It has the highest concentration of organic matter and microorganisms and is where most of the Earth's biological soil activity occurs.-Importance:...

 and Subsoil

Subsoil

Subsoil, or substrata, is the layer of soil under the topsoil on the surface of the ground. The subsoil may include substances such as clay and/or sand that has only been partially broken down by air, sunlight, water, wind etc., to produce true soil...

. The parent rock has little organic matter.