That indicates bad sectors that have not yet been mapped out for nonuse, and they are an issue if they keep happening the drive is bad. You might just see some intermittently, and then they will be remapped and the reallocated sector count will rise and that sector will not be used again. If the reallocated count keeps rising that also indicates a bad drive.
To clean up the drive and find out if it stops having bad sectors you can download DBAN ( http://www.dban.org/ ) and then reformat and quick erase in DBAN, which can do a zero write. Then all bad sectors should be gone. Any new ones that pop up fast indicate a bad drive.