Its an attribute that most people dont possess. Its your heart telling you to do something that is right, even though it might me unpopular, or it dosent go with the flow. BeckyCole is right. Its courage under fire. It sometimes makes you stand out among the others. Its not a learned conviction, its either you "got it" or you dont. It gives you that extra edge when peer pressure is knocking.
True Grit is generally an expression that is mostly used in the southern United States and it refers to the strength of mind. It also means the courage to see through one's convictions even in the face of strong adversity.
At one time, when people said someone had 'grit' it meant that person had courage and strength of will and stuck to something when things got tough. They were tough and tenacios and brave in the face of adversity.
I'd say that having "true grit" is about the same as being utterly relentless. A person like that always moves forward against opposition, and can't even understand the concept behind the word 'quit'. Ever see the movie 300? King Leonidas in that movie IS true grit, so when you're wondering about what it is, just think "Madness? This Is SPARTAAAA!"