Scrubbing is more like reading the block, calculate the checksum. It's the same as stored for the block (not at the block, important difference)? Nice. Not the same checksum. Go to the mirror and fetch the correct one, respectively use an algorithm to calculate the missing block from the raidz1/2/3 set and write it to the disk with the error.
I see. From the description it's quite obvious that ZFS handles most of the fault cases automatically. Still admins might want to force it to recheck all the blocks, this command achieves exactly that. Frankly I don't know what exactly fsck does and I guess it's different for every FS. For ZFS scrubbing seems to be an adequate alternative.
Hence if anyone asks you whether ZFS has fsck it's easier to say yes it does and it's called scrubbing.
