Moisture Sensitivity Level 3. Meaning (according to the standard) that the part must be mounted within 1 week of being taken out of its sealed bag, and is only good for 1 year inside the sealed bag. Otherwise it requires a baking process to drive out any accumulated moisture. This is to prevent internal damage when the moisture expands during soldering. It applies mostly to high density chips like 32-bit ARM. Not so much for 8-bit.