If the drive's controller detects a problem with a disk access, it swaps that area of the disk out and replaces it with one from a set of spares (built-in redundancy). When the spares run out, your data is at risk. The main reason for this reallocation is to overcome manufacturing defects which would otherwise trash the majority of the manufacturers output, but it also provides a useful self-healing mechanism for the warranted lifetime of the drive.
Random faults will occur from time to time. There should be no pattern to them, and a drive that reports occasional reallocations is not a worry. It is time to worry when large batches of reallocations are occurring - the drive surface is degrading, and data is on borrowed time.