Basics

You can learn a little bit about Migrad by running the following in a ROOT commandline:

TMinuit m
m.mnhelp("MIGrad")

This outputs the following documentation:

***>MIGrad  [maxcalls]  [tolerance]
 Causes minimization of the function by the method of Migrad,
 the most efficient and complete single method, recommended
 for general functions (see also MINImize).
 The minimization produces as a by-product the error matrix
 of the parameters, which is usually reliable unless warning
 messages are produced.
 The optional argument [maxcalls] specifies the (approximate)
 maximum number of function calls after which the calculation
 will be stopped even if it has not yet converged.
 The optional argument [tolerance] specifies required tolerance
 on the function value at the minimum.
 The default tolerance is 0.1, and the minimization will stop
 when the estimated vertical distance to the minimum (EDM) is
 less than 0.001*[tolerance]*UP (see [SET ERRordef]).