In Sons of Anarchy, "shot caller" refers to a gang's veteran leader. It's the person who "calls the shots".
The term "shot caller" refers to veteran leaders of the Aryan Brotherhood also known as The Brand, one of the most elite prison gangs in the world. They are groups of white inmates that were formed in the California prison system in the late 1960’s.
While the exact number of Aryan Brotherhood members is not known, the gang has chapters in almost every major state and federal prison in the country. Estimates of the brotherhood's total strength vary, but nearly all exceed 15,000 members and associates nationwide, with roughly half in prison and half out. Learn more here.
Of interest, WhatCulture.com shares 10 Moments That Proved Sons Of Anarchy Was The Best Show On TV.