It's a 'best by' date, which just means that the manufacturer won't guarantee quality past that date.

Over time the salt crystals will fuse together (form clumps) because of moisture in the air. Sugar does the same thing. The clumps can be easily broken up and are still perfectly edible, but clumps in new product would be considered a quality issue.

Edit: this is an educated guess as what that best before date means, but I'm actually not a 100% certain. I'm not from the sector.

The comapny just wants you to throw it out and buy more if you haven't used it fast enough for them.

If enough humidity over time gets in there, the salt can start caking and forming larger crystal clumps. However, the salt itself isn't damaged by that process and will work fine if broken back up and used in the quality you need.

A best by date here would be a notice from the manufacturer that the product should be shelf stable at least that long before "degrading".

Quiet you. These artificial expiration dates are the only way I can ever clean out the pantry without my SO freaking out.