Trial By Fire
In their meager beginnings, Shadowrunners are green and unproven. The victories and setbacks involved in life, and their business, eventually teach them new skills and improve on their existing skills. They may begin to overcome flaws, strengthen their minds and bodies, and improve upon their natural talents.
The experience they draw upon to do so is called Karma.
The rules for improving characters with Karma may be found in the Core Rules (p. 107).