Xenoliths are fragments of rock that become trapped within another type of rock during its formation. They often originate from the surrounding rock, known as the country rock, and can provide valuable information about the geological history of an area. These inclusions can vary in size and composition, offering insights into the conditions under which they formed. Typically found in igneous rocks, xenoliths can help scientists understand processes like volcanism and magma movement. By studying these rock fragments, geologists can learn about the temperature, pressure, and chemical environment of the Earth's crust at the time of their incorporation.