A reallocated sector count is where a few sectors of the drive to store data can no longer be used, because they are flagged as damaged.
Every hard drive has spare sectors for redundancy or error correction.
But you only get a set amount of extra sectors to combat errors or sectors of the drive that fail.
Once used up as the drive gets more errors then you loose disk storage capacity and data after that.
You will receive read write errors eventually meaning the drive needs to be replaced to keep a good data integrity. Reallocation is a way of moving a bit of data before the sector fails, and is copied to another part of the drive in an attempt to keep the data.
It can also be a miss calculation, of free space or space taken up of data on the drive, and is corrected.