My textbook also says that the sample s^2 = (1/n-1)[∑(y_i - y bar)^2] has n-1 in the denominator because it has n-1degrees of freedom.

And s^2 = (1/n-2)[∑(e_i)^2] has n-2 in the denominator because it has n-2degrees of freedom.

Now I am puzzled...what is "degrees of freedom"? Why does it have n-2 degrees of freedom? What is the simplest way to understand this?

Thanks!