They do not model sizes of sets the way that natural numbers do. The Real Numbers . These are abstract concepts that describe measurements of continuous quantities , such as length, weight, quantity of fluid, etc. The Complex Numbers . Thus, the complex numbers are a number system in which there is an object whose square is -1.
Complex numbers are extremely important mathematically.
They have less direct relevance to real-world situations, since they aren't measurements of single quantities. More information on this is available.