The population mean of y. And if it doesn't make a lot of intuitive sense yet, well one, you can just always kind of think about what it's doing, play around with some numbers here. But the reality is, it's saying how much they vary together. So you always take an x and a y for each of the data points. Let's say you have the whole population, so every x and y that go together with each other, that are a coordinate, you put into this. And what happens is, let's say that x is above its mean when y is below its mean. So let's say in the population, you had the point, so one instantiation of the random variables, you have, you sample once, I guess, from the universe.