When an error is encountered, a script can either continue with the next command (if possible), or display an error message and stop. By default, it will display an error message and stop. During the script, you can switch from one mode to the other, as often as necessary.
Forces a script to keep going if an error is encountered (if possible).
Forces the script to stop and display an error message when an error is encountered. This is the default. You will probably want to use the default until you are sure there are no more bugs in your script.