You'll need to know Objective-C or Swift, XCode, Cocoa Touch touch design components, and the functionality of your app needs to be broken down into different "scenes". The UIKit library is full of advanced graphical features such as buttons in a button bar with images that transform upon launching an app.
Also, Apple requires that all apps in the App Store be approved by Apple. Finally, it can be hard to optimize for just one device when your app needs to work on other devices down the road as well.
Additionally, iOS development is more complex than Android development because they have less support for third-party libraries and tools like ADB or LogCat in lieu of XCode's debugger over UIAutomator and XCODE UI Tests. So a lot of manual testing has to go into an application before release which doesn't happen in Android Development as much.