What is the difference between launchscreen.storyboard and main.storyboard

They are two completely different things. The launch screen is what first appears when the user taps the app icon before the app is finished launching. It shows a single, static screen. It can't be dynamic and it can't use any custom classes or code. It's the replacement for launch images.

The main storyboard is what your app actually displays when the app is running. It contains your app, code, and logic.

When you run your program in Xcode simulator gets open and your application gets open then you can see black white screen for fews seconds before actuals developed screen

Blank screen - LaunchScreen.storyboard - Non editable

Developed Screen - Main.storyboard - Editable for which you can able to make changes

The launchscreen.storyboard (basically a loading screen) is what comes up when the user loads the application. How long it appears to the user depends on how long it takes for the user to load your application. You can change the duration of the launchscreen.storyboard but it is not recommended, generally users want the app to load as fast as possible

The main.storyboard is appears onto the screen AFTER the launchscreen.storyboard