See the section in the polyfit (link) documentation on Output Arguments, specifically R (link). It is the ‘Triangular factor from a QR decomposition of the Vandermonde matrix of x’.
Since ‘R’ is a field of ‘bint’ (in the example you posted), there is nothing further you need to do, since ‘R’ is calculated for you. (If I remember correctly, the ‘R’ field is the covariance matrix. It has been a while since I explored polyfit.)
