F-ratio: the ratio of explained to unexplained Mean Squares (MS), where the numerator is the MS explained by the model and the denominator is the error MS left unexplained by the model. A significant effect is indicated by a value of F that exceeds the critical F for a predefined Type-I error.
The Mean Square is the variance measured as variation per degree of freedom.
Doncaster, C. P. & Davey, A. J. H. (2007) Analysis of Variance and Covariance: How to Choose and Construct Models for the Life Sciences. Cambridge: Cambridge University Press.