"The mean square of the difference between the data in the sample and the sample average is called sample variance."

Now the concept of "sample variance" in statistics is represented by "var", which has become a kind of "language" when we compile formulas (such as OPEN is the opening price, CLOSE is the closing price, HIGH is the highest price, LOW is the lowest price and so on). It is neither necessary nor possible to understand a basic mathematical concept in isolation. We just use it to compile a meaning we want to express into a formula in a prescribed "language" for my use.