Variance formula S-square = ∑ (xi-x) square / n-1 What is Xi? How to calculate it?

Variance formula S-square = ∑ (xi-x) square / n-1 What is Xi? How to calculate it?

What you give is the variance formula of the sample. It means to subtract the average value of the sample from each value of the sample, then add the squares, and then divide by the number of samples minus one, so Xi is every value in the sample
For population, the formula of variance is: square of ∑ (xi-x) / n