Windows 10/64 + matlabR2018a

Hello, there is a PSD estimate of matlab for:

The function [Sxx]=estimate_psd (X, lambda)
Sxx=filter (1 - lambda, [1 - lambda], abs (X). ^ 2, [], 2);

The authors note in the program lambda is "smoothing factor for PSD estimation" is what meaning? How should choose?