[shmageggy]

It begins training the network using the backpropagation algorithm.

> Next, the network is asked to solve a problem, which it attempts to do over and over, each time strengthening the connections that lead to success and diminishing those that lead to failure.

On each iteration, it calculates how bad the predicted output is, then adjusts the weights between neurons to lessen that value. Google backpropagation for more info