Apple’s App Store is a marketplace for iPhone and iPad users to download paid and free applications and games for their devices. Although, the majority of contents on there are free, these applications do offer something called In-App purchases. Basically, it is a hidden cost in “free” apps. iOS developers can use this opportunity to make money while putting their original work on the App Store free of charge.
Some game companies do include In-App purchases into their free applications and those users who buy it will unlock extra items or rewards (more maps, players, weapons, no ads, etc). Sometimes, in-app purchases aren’t that good like it suggested, to avoid wasting money on premium contents that you don’t like, why not try it out for free first.