What exactly is "Sushi Grade" fish?
"Sushi grade" means that it is safe to prepare and eat raw.  In order to do that, it must be frozen to kill any parasites.  That means it either has to be:


Frozen at -20° C (-4° F) for 7 days; or
Frozen at -35° C (-31° F - "flash frozen") for 15 hours.


There aren't any official regulations about the fish itself or its quality, and most sushi/sashimi distributors have much more stringent rules of their own beyond the freezing guarantee.