The character is the absolute best at what they do.
The character may reroll failed skill tests for the chosen skill (but only once for a given test). This talent does not stack with other talents that would allow rerolls. Grandmaster can be purchased multiple times for different skills. If this talent is taken with a magical skill, then it increases the bonus to effective skill rank by +1, which allows for the casting of Grandmaster spells, the enchantment of Master spell effects, or the creation of level 6 potion/poison effects without penalty. The level is determined by the governing characteristics of the chosen skill.