I was asked this question when Emma and I were on our way to the polls. Simple answer for a 4 year old: It is when people decide who will be president.
Emma: What is a president?
Me: the person who is the boss of the country
Emma: why do you vote?
Me: so we can decide who will be the best person to be the boss
Emma: I think you will be the best boss, Mommy.
*sigh* Thanks Emma. I think I would be too. Certainly better than what we have to choose from today.