Made when sediments fill in a cavity made when an object decayed and the sediments harden into rock

A fossil is formed when a plant, animal or other organism dies. The soft tissues of these organisms gets decomposed and degraded by decomposers present in the soil. The remaining hard and undecayed part like wood, shells, skeleton are left. These remains are covered by layers of soils. These layers of soil gets compacted to produce rocks. The rocks exhibit the dead remains are called as cast. A cast is also termed as mould fossil. It is the general form of the original organism.  

Therefore, the description: Made when sediments fill in a cavity made when an object decayed and the sediments harden into rock is cast.