How to segment image using SOM?

Matlab has SOM function

selforgmap

Does anyone know how can use this function to segment the below image, for example, to 3 classes?

I tried to do this using the below code:

clear;
fix = zeros(500,500);
i = ones(500,100);
fix = double( mat2gray( [fix(:,1:200) i(:,:)*255 fix(:,1:100) i*125]));

%%SOM

net = selforgmap([500,1],100,3,'gridtop','dist');
net = train(net,fix);

y = net(fix);
classes = vec2ind(y);

Actually, I do not fully understand it.

Thanks