sigmoid function and derivative

[Sharan Basappa]

Folks,

I know this is not a machine learning forum but I wanted to see if anyone can explain this to me.

In artificial neural network, I can understand why sigmoid is used but I see that derivative of sigmoid output function is used. I am not able to understand why.

For example:
# convert output of sigmoid function to its derivative
def sigmoid_output_to_derivative(output):
    return output*(1-output)