Can someone help me understand this line of code?

Actually he never specifies a target during the example you point to. The name of the constant is just there to make it obvious what it will end up storing. JavaScript itself never does anything special based on what you name something, he could have called it abc and the code would have worked identically.

The reason he doesn't specify a target is because there is no need to. The function he is writing is triggered when a change event is emitted from within the ul, the only element within the ul that can emit that event is the checkbox, so if the function runs you already know you are dealing with a checkbox.

If the page was changed so that there where multiple elements within the ul that could trigger a change event and you wanted different code to run based on the specific element, then you would need to use an if statement to check exactly what element triggered the event. Like he has done in previous tutorials.