According to the function documentation, those square brackets contain any output arguments. Because the square brackets you show in your question contain zero variables, that function has zero output arguments.
function [A,B,C,D] = myfun(...) % 4 output arguments
function [A,B,C] = myfun(...) % 3 output arguments
function [A,B] = myfun(...) % 2 output arguments
function [A] = myfun(...) % 1 output arguments
function [] = myfun(...) % 0 output arguments
For one or zero output arguments the square brackets can be omiited entirely:
function A = myfun(...) % 1 output arguments
function myfun(...) % 0 output arguments
