If array2 is a logical (binary) array then it takes those elements from array1 that are 1 in array2.
For example
array1 = magic(5)
array2 = false(5);
array2(2, 3) = true;
array2(3, 1) = true
x = array1(array2) % This will be a vector, not a 2-D array.
So see how it took the (3,1) and (2,3) element and strung them together into a vector?
On the other hand if the array2 is an integer array, it will use those as linear indexes into the array 1.
The linear indexes are in column major order (down rows first, them move across columns):
1 6 11 16 21
2 7 12 17 22
3 8 13 18 23
4 9 14 19 24
5 10 15 20 25
So if array 2 was
array2 = [2, 7; 17,22]
x = array1(array2)
It will take those elements from row 2 that had those linear indexes.
