rcond برمیگرده به الگوریتم LS و مقادیر ویژه ماتریس ها دقیقش رو باید برید تو توضیحات این الگوریتم بخونید
polyfit issues a RankWarning when the least-squares fit is badly conditioned. This implies that the best fit is not well-defined due to numerical error. The results may be improved by lowering the polynomial degree or by replacing x by x - x.mean(). The rcond parameter can also be set to a value smaller than its default, but the resulting fit may be spurious: including contributions from the small singular values can add numerical noise to the result.
خط sp.sum((f(x)-y)**2) هم دقیقا ترجمه مجموع مربعات خطاست