Ordinarily hard disks operate transparently. Any problems are handled by the drive logic.
Disks in use may find problem sectors or tracks and they then map spare area into the block number table when the error correction cannot cope.
badblocks -sv -b 512 <device-name>
If the disk is one of the newer 4K sector designs, change the 512 to 4096.
Pending sector count is simply the number of problem areas that the disks is considering for remapping. This areas may continue to be fine but the error level may be slightly higher than the design threshold.
We consider that disks with pending sectors to be operational but we use them only backups which can force the disk to fix sectors when data is written.
Its advisable to use multiple redundant backups just in case.