I am looking to find out what reindexing means.
I recently sent back an AR-15 upper to bushmaster for accuracy problems. When it came back from Bushmaster the repair slip stated that it was reindexed. I am just looking to find out what this means.
thanks

It means returning things to zero. In your particular case it probably means that the barrel was turned enough to align the front sights a little better with the rear sight and/or the uppers sight rail. This also places the gas port in alignment with the "thru hole" in the delta ring and the receiver.

Standard sight towers are attached by way of tapered pins and usually they are well centered and aligned, but occasionally a sight will come a little out of whack and will need adjusting (reindexing) to bring things back into alignment.