If you don't understand the meanings of the options you probably should NOT use this tool OR check the Quick-Connect Guide!
Besides the obvious and topics covered in the other sections there are however some things to point out:
Unless you know exactly what you are doing, you should use the "URL" field with the DNS of the server.
In 99.99% you do NOT want to target the IP!
Whatever you do - hit "Lock ON" BEFORE you start.
Choose an attack method from the pulldown and see which fields get enabled.
You can change the speed slider on the right during the attack.
If you use ReCoil or slowLOIC you can change the amount of threads during the attack as well.
The interesting value here is "Failed"! If this goes up you are doing it epically wrong (o.O) or your target is DOWN == EPIC WIN!