the first stage as we discussed is the convolution operation, here multiple feature maps are produced 
by performing several convolutions. In the second stage of CNN, we need to transform the linear feature maps 
obtained from the convolution operation into a non-linear representation. This is achieved through the 
application of an activation function. In most cases, we use ReLU (Rectifier Linear Unit) as the activation 
function in the activation layer thus this layer is sometimes known as ReLU Layer.

relu = max(0,x) ;output from 0 to 1 and ignored -1 or -0.0001