Function histeq source code in matlab.

this is a line of code that I see in the source code of histeq: tol = ones(m,1)*min([nn(1:n-1),0;0,nn(2:n)])/2;

Can any one explained what is this line of code do ? What is the explanation for this code?

Parameters:

m = 4

n = 256

nn is 256 x 1 matrix, class double