From Google and other companies HDD failure prediction research, reallocated sector count is one of the most reliable drive failure indicator. In the Google paper I read a few years ago, they said appearance of the first reallocated sector has the strongest correlation with drive failure within the next few two or three months.
The way reallocated sectors work is that if the drive is no longer able to read a sector due to physical damage to the recording media, it will remap the sector to its spare table. When this happens, the data in the damaged sector is usually lost - corrupted beyond repair as far as the drive can tell.
Start backing up your data elsewhere before any more of it gets corrupted or lost when the drive decides to fail completely.