That means they replaced hardware inside the phone. If the phone didn't power on, then they probably replaced something that has to do with the power button powering it on, or anything else that would cause the phone not to power on.
This means they updated your roaming list. This isn't a bad thing, and could actually speed up your 4G. I recommend doing this every few months. You can do this by dialing *228 from your phone, and pressing option 5 as soon as the automated service starts speaking.
This means they updated your firmware from 4.1 to 4.3. However, I recommend now a backup and a factory reset when you get your phone back as they probably didn't do that and it will solve issues you may have after the update.