Fertility

How much of a difference is there really of your chances of getting pregnant during your "fertility window" and not? I notice this app visually shows you your fertile window. But then does that mean all other times it's time to sex it up because you won't get pregnant? 
.... Help.. From a new young girl entering the world of sex and trying to understand the complications of it all. Obviously I am not the most educated int his topic and so I am seeking out for help!