How to get a covariance matrix is well defined, but I don't really know how to use it once obtained. I'm trying to find the best parameters for a data set with a given function. I'm having four parameters a1,a2,a3,a4 and from these parameters I have the covariance matrix. I'm supposed to get the 'errors' of the parameters from the matrix. I know that the covariance between the same element gives the variance so the elements on the diagonal of the matrix (when i=j) give the variances. I can't really figure out what the variance means, is it an absolute error in the parameters or maybe a relative error? Or maybe it's something quite different, can anyone explain me what the variance means?