It means he will completely reformat the drive and reinstall your operating system. It will like starting with a brand new machine. I've found this is what many computer repair places do because it's just so much simpler that way. Plus if malware is the problem, then they know it's been totally wiped out.
Is this an XP computer? If it is, then you might ask if it's worth restoring XP since Microsoft will be ceasing all support in just a couple of months.
In my PC corner for the winter, gaming and knitting.