Meso Generators are a type of scam created to steal your Maple Story account. They should be avoided at all cost - even Nexon has put up warnings against them on their website. There are actually a number of different websites with "Meso Generator"s on them. Most of the sites are easily detected by bad grammar, lack of organisation and lack of persuasive argument. Do not be tricked by these phonies.
Meso Generator is also a term for a type of hack used by hackers in order to hack their character's meso total and (usually) make it higher. Certain "Meso Generator" hacks also have the feature to boost characters stats. These hacks are illegal and to be avoided.