Does the "use it or loose it" principle apply to the brain?
Yes, brain power is eroded via a lack of practice. This occurs through the processes called synaptic pruning and brain plasticity.

I will leave you with a very basic answer as I am unsure of your level of understanding of cognitive processes. First, you must understand neurons. Then, you can begin to understand synaptic pruning and brain plasticity. Finally, here is a link to a bunch more information and brain plasticity exercises.