Moisture Senstive Devices - Shelf Life
The concern is that moisture may enter the device package.  When the "moist" package is heated during reflow soldering, that moisture could turn to steam, and damage the package as it expands.  If you will be mounting the parts using reflow soldering (or otherwise heating to near solder temperature) they should be baked gently first, to dry them.

The functionality of the parts should not be affected.