I am new to machine learning and could understand the simple perceptron. I came to a formula while reading about perceptron.
$$ y = \sum_{k = 1}^{K}z_{k}(x)(\Theta _{0}^{(k)} + \sum_{j = 1}^{m}\Theta _{j}^{(k)}x_{j}) $$
where $$ z_{k}(x) \epsilon R^{+} $$
I could understand the inner block represents simple perceptron. and $$ z_{k}(x) $$ is a element from real number. What is exactly $$ z_{k}(x) $$ representing here. Could you give a hint ? Is it changing single layer perceptron to multi layer or other ?