A full-back (Left-back or right-back) is a defender who takes up the holding wide positions and traditionally stayed in defence at all times until a set-piece occurs.
A full-back is usually positioned in the right-side or the left-side of the field based on their preferred foot. They are required to have good attributes for defending, tackling, marking, pace and vision.
These are the roles that are usually assigned to Full-backs:
Typical defensive roles
Jockeying and smart positioning to bother opposing wingers/attackers