Normally when I try to describe that issue to someone non-tech, I tell them think of a car trying to get to a destination, if it doesn't have enough gas then it's not going to make it there without getting more gas, or if it's an electric car recharging the batteries.
For example if I'm trying to Los Angles (from Orlando) I need a set amount of gas to get there, but if I don't have that amount of gas I'm not going to get there. So I have insufficient resources to make my request.
Hope that helps.