Replacement sectors are used by hard drive manufacturers to compensate for errors in the manufacturing process.

Before a modern hard drive leaves the factory, it is low level formatted*, this does a number of things, including prepare the disk to be used with a certain disk controller. During this low level format, there are always some bad sectors detected. To prevent selling disks that have a bizzare, lower than advertised capacity, manufacturers always make their disks with extra sectors. These extra sectors are logically inserted in place of the bad sectors, to bring the disk up to the required capacity.

There is not really anything the average computer user can do to remap these sectors without specific software from the manufacturer, and there is really no reason to do so. If bad sectors start showing up to the extent that they are interfering with the use of a drive, it is probably time to back up the data and move it to a new drive.
*ST506 and ESDI drives had to be low level formatted by the user, but they are no longer used, having been displaced by IDE and SCSI.