A model in which a developer earns money for showing ads. A developer can be paid per impression, per click, or per action. You can choose one of the popular ad platforms, like AdMob, MoPub, Appnext, Mobvista, etc. It can be combined with other monetization models as long as not it’s overused – otherwise it may cause user annoyance and lead to uninstalls
In addition, you don't pay app stores their fee for this, unlike other monetization models.