What is iOS?

iOS is mobile operation system developed by Apple Inc. Apple’s devices(iphone, ipad) run on iOS

What i need to begin iOS app development?

1 Mac:
you should get a Mac system with Mac OS X (version 10.11 or higher)

2. Xcode:
Xcode is the only IDE for app(iphone or ipad) development.

3. Simulator/ iphone

What programming language i should learn to start iOS app development?

you should learn Swift or ObjectiveC.

What iOS developer should know?

1. User interface design using Xcode
2. Cocoa Touch
3. Json parsing using swiftyjson or HandyJSON.
4. Rest call using alamofire