This tool basically sees users entering the serial number of their iPhone 6s and checking it against the database. If your serial number is part of the replacement program, you will be able to take your iPhone into the store and get its battery replaced for free. This seems to mostly affect devices manufactured between September and October 2015, so if you got your iPhone 6s much later, chances are you’re safe.
However it should be noted that devices need to be in good working condition in order to receive a free battery replacement. This means that any damage to the phone like a cracked screen means that you’ll have to repair that first before replacing the battery. We’re guessing Apple wants to avoid any complications of customers accusing them of damaging their products, or further damaging the iPhone during the course of the battery replacement.
In any case if you want to check to see if your phone is eligible, head on over to Apple’s Support page and enter your phone’s serial number. If you’re unclear on how to find your serial number, you’ll find the instructions on Apple’s website here.