What does the sigma value returned from Friedman's test represent?

When I run the following for a Friedman's test:

 M = [0.7876    0.7219    0.7743    0.7938
    0.9045    0.7142    0.6346    0.7114
    0.8912    0.8252    0.7809    0.7675
    0.9031    0.7122    0.6132    0.6758
    0.8296    0.7334    0.6563    0.6878];
 [p tbl Fstats] = friedman(M)

I get:

 p =

    0.0293

 tbl = 

    'Source'     'SS'         'df'    'MS'        'Chi-sq'    'Prob>Chi-sq'
    'Columns'    [15.0000]    [ 3]    [5.0000]    [9.0000]    [     0.0293]
    'Error'      [10.0000]    [12]    [0.8333]          []               []
    'Total'      [     25]    [19]          []          []               []

 Fstats = 

       source: 'friedman'
            n: 5
    meanranks: [3.8000 2.6000 1.4000 2.2000]
        sigma: 1.2910

I understand where the values in tbl come from and what they represent, but I can't figure out what Fstats.sigma is supposed to be. Does anyone know?