What is MSL?
MSL is JEDEC regulation. This regulation is established to prevent products such as semiconductors from being damaged resulting from volume expansion by moisture evaporation at reflow etc. Moisture is mainly absorbed by resin used to seal the package of semiconductors etc. There is Moisture Sensitivity Level corresponding to the risk of damage in the regulation. It is needed to take measures corresponding to the level to pack with a desiccant and perform baking treatment after a certain period of time passes since unsealed etc.
Because the damage by moisture absorption is mainly caused by resin used in electronic parts, ceramic capacitors are out of this regulation as of september 2014 since they are resin-less.