LDL-cholesterol is a lipoprotein that is also usually called the "bad" cholesterol. The name "bad" cholesterol comes from the fact that LDL has a high content of cholesterol and also a tendency to release cholesterol on the way. This, in turn, means that there may be more cholesterol in the blood than is needed, which leads to the cholesterol getting stuck in the vessel walls and causing cardiovascular diseases.
LDL cholesterol is a risk marker for cardiovascular disease. By measuring your LDL cholesterol, you gain insight into your values.