It's an indication of the amount of read errors the drive has experienced.
Can't say what number "should" follow it because it depends on the manufacturer of the drive, but generally lower is better.
Also don't be surprised if it's not 0, all hard drives experience read errors, its unavoidable due to the amount of data they cram into such a small area, it only becomes a problem if the errors are above the value the error correction can deal with.



