You can execute one step at a time in debug mode. This is called single-stepping. It is useful if you want to examine the execution very closely.
You can single-step when Design Studio is ready to start a new debug, or when it has stopped during a debug. To execute the next step, click the icon. When that step has been executed, execution will stop. You can then click the icon again to execute the next step, and so on. At any step, you can also resume normal execution by clicking the icon.