Roll bone controls in the Attribute Editor let you define how you want a character's roll bones to behave as you rotate the character limbs. See also Role nodes.
For example, if there is a Left Forearm Roll bone defined, the Left Elbow Roll property controls the distribution between the left forearm roll and the elbow. Setting the Left Elbow Roll slider to 0.500 means that 50% of the left elbow roll will be distributed between the left forearm and the left elbow roll as you rotate the left arm.
(If there is no Left Forearm Roll bone, then Left Elbow Roll controls the distribution between the left hand and the elbow.)