Understanding is one of the three intellectual virtues, and it is a great gift. For example, it might be self-evident that the sun rises each morning in the east, but why that occurs is something that would need to be studied. Things that are known in and of themselves, those things that are self-evident, are known as principles.
Understanding is the ability to seize upon principles.
Not everyone has the same degree of understanding. There are people who have an aptitude for math, and there are those who can't count without their toes. Those who have the aptitude for math have an understanding of the subject. The principles come easy to them.