Of all the statistics of a character, we'll be the most vague when describing mana, since most of its uses are secrets that will be revealed through the ongoing story of the game. But the short version is mana is the strength of a character's metaphysical spirit, an ethereal quality that can be represented in faith, a soul, a life force, or a number of other belief systems. As a scale:
0: A soulless husk, probably shambling around while hungering for brains.
1: Listless, unemotional, and demons seem ambivalent about getting their soul. Not much of a bargain there.
2: Average human. They have moments of a vibrant spirit, and moments where they just want to lie around all day.
3: The annoyingly energetic.
4: Strange things happen around them. Pretty unlikely they are an atheist.
5: Calm down, kwisatz haderach.
6: Not so much having a belief system as inspiring one.