How to find Primary voltage & the Main stress direction.

I try to find "primary voltage" and "the main stress direction." (I think they call, or elese it could be a bad translation)

And I find this example, and I understand it but not all of it.

M2 = eye(3); M2(1:2,1:2) = [1 2; 3 4];
[V, D] = eigs(M2)
V =
   -0.4160         0   -0.8246
   -0.9094         0    0.5658
         0    1.0000         0
D =
    5.3723         0         0
         0    1.0000         0
         0         0   -0.3723

M2=eye(3) told us that is 3x3 matrix.

And M2(1:2,1:2) = [1 2; 3 4]; I do not really understand, but I can see that [1 2; 3 4] are the vectors of the matrix. What M2(1:2,1:2) do?

[V, D] = eigs(M2), there D gives us the Diagonalization and V gives us ...? I don't know.

Hope you can help me out there.

Regina