in context of Artificial Neural Networks:
For a 'neuron' to learn something, there must be:
Then we can analyse how this change affected 'neuron', and how closer or further away it is from it's goal, a desired behaviour.
With enough of time, with analysis, with a certain degree of control of changes, changes might become more rapid, bigger, smarter, better.
See also: Neural Networks, Stitie Space, Simple Artificial Intelligence.