New developers joining the Apple Inc. development ecosystem must have landed on something like LLDB, which looks like an unexpected error and quits the execution. However, if observed closely, your code had no errors and program ended unexpectedly with a highlighted line in ViewController.Swift file (Depends on your application) was due to a breakpoint you might have added accidentally. Remove the breakpoint, and code would run fine as usual.
If you observe the editor closely, you will find an icon with a bold arrow shape pointing to the line at which break point has been enabled. To run without any issues,
1. Check if there are any breakpoints stopping the execution.
2. Click on the little blue icon so as to disable the breakpoint, or else you can right click and delete or disable the breakpoint.
3. Try running the app again, it would be flawless. See the image for more reference.
Coming to the yet another concept, What is this LLDB?
LLDB is the Apple answer to the GDB, and has been developed ground up with LLVM compilers to help the users with advanced debugging capabilities. LLDB was first introduced in the Xcode 5 and has the commands just similar the GDB, making the debugging and flow control extremely easy for the developers.
Hope this solves your issue, comment if you have any query!
Operating System: El Capitan Beta 4
Xcode Version 6.4 (6E35b)
MacBook Pro Retina mid 2014.