Requests and impressions

Hi, I don't fully understand the concept of requests and impressions. When a view with an iadBannerView loads it makes a request to show an add, and when/if the add is shown it is an impression. I don´t get money of requests, but impressions, right? What kind of things could prevent the request to turn into an impression? The user navigating to a different viewController before the ad is loaded? Bad network? Would for example disable userInterAction for the app until the ad is shown be a way to optimise the fill rate?