Hi,
The makeErrorBand computes the upper/down uncertainty in the curve. It needs a vector of curves representing a random sample of variations of the parameters. The parameter variations are obtained by sampling from the fit covariance matrix result.
Otherwise you can provide as input the upper and lower variations of the curve versus the parameters (so two vectors which have as size the number of parameters) and in addition the fit covariance matrix.
You can see how the methods are used internally in RooAbsReal::plotOnwithErrorBand
https://root.cern.ch/doc/master/classRooAbsReal.html#ace8c056713a182f534d3fa144895d37c
Lorenzo