What is the meaning of in swift

|| Called Logical OR Operator . If any of the two operands is non-zero, then the condition becomes true.

Put simply, @UIApplicationMain is an indicator for Swift applications that indicates which object is your application’s main application delegate file. In Objective-C application templates, you would have a trivial main. m C file which sets the application delegate.

_ : The under score operator is usually used in swift function definition. It is used to replace the function parameter’s external label.