Think of meso guard like magic guard but instead of taking away from mp it takes away from your mesos.[/quote]
Basically this. If I remember correctly (haven't bothered checking description since pre-BB) it halves the damage you take, and 61% of the halved damaged is deduced directly from your mesos (if you're at 0 mesos, you can't go negative, but mg's effect will cease to work). It actually saves mesos.
i'm a lazy bastard and tired of saying this lots of times, so i'm just going to link past explanations i've said. here and here
and for some added info. here and here in case anyone is wondering.