Prism quantifies goodness of fit by reporting the standard deviation of the residuals, written Sy.x by Prism (but sometimes called Se). Remember that the residual is the vertical distance (in Y units) of the point from the fit line or curve. If you have n data points, after the regression, you have n residuals.
Sy.x is expressed in the same units as Y and all can be interpreted as the typical deviation of the points from the line or curve. It is computed by the equation below, where n is the number of points and K is the number of parameters fit by regression (usually 2, but 1 if you force the curve through a point). The value n-K is the number of degrees of freedom of the regression.