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I'm assuming that is just the composition of the two linear operators, in which case that is matrix multiplication. In other words, the definition of matrix multiplication comes from viewing the two matrices (x and y) as linear operators and composing them.
If it is something more than that, then I think we need more context.
Then y contains either linear or logical indices:
x = reshape(1:9, 3, 3); y = x; x(y) % same as x
y = ones(3, 3); x(y) % Repeated first element of x
y = true(3, 3); y(2,2) = false; x(y) % All elements without the central value
y = x + 5; x(y) % ERROR, the linear index must be 0 < index <= numel(x)