When you run your project, it opens up in the Simulator or the device (mainly in the simulator). You can stop it from running with the stop button. Clicking on the stop button returns you to your code. You can then update your code and rerun the project. Everything is fine but, if there was an error while the project was running, it goes back to Xcode and displays a source, either your own source code or some other code where the error occurred. However, instead of waiting for errors to break your code and provide you with an opportunity to debug it, you could set breakpoints. Breakpoints are, as the name suggests, points in your code that cause the running code to break (go to debugging mode) when the execution reaches that point.

One of the ways in which most developers try to resolve errors is by including statements that output some text and check what happened or is happening. This could work for some. However, these will have to be removed when you build your application...