Main use of "break" command is to halt execution of script until user resume it again manually. Supposing if you want to take some actions manually at any specific stage of your currently running script, then you need to halt your current execution and you can do it with "break" command. See bellow example for better understanding.
Here first it will open "https://www.google.com" URL in browser. Second command will halt selenium IDE execution and now it will show "Pause/Resume" button enabled in selenium IDE window. Your script's execution will remain halted until you manually resume it by pressing "Pause/Resume" button on selenium IDEwindow. During halt period, you can take any action on you browser. Here keep in mind one thing - you have to resume script execution manually it will not resume automatically.
"goBack" command same like you press "back" button on your browser. In above example, forth command will virtually click "back" button of your browser on "https://www.yahoo.com" to open previous page "https://www.google.com".