Barrel Roll is a maneuver used to evade missile attacks. It causes your Gear to make a complete 360 degree spin left (if A) or right (if D). If executed at the right moment, missiles tailing you would 'pass through' as your Gear rolls sideways.
A Barrel Roll can be executed by rapidly pressing the "A" or "D" key twice. It costs 3 SP per roll and has a 3 second cooldown before another roll can be executed. This skill is innate to all Gears from the start and does not need to be bought from the Skill Shop. However, an A-Gear cannot execute a Barrel Roll while hovering on the ground.
There is an I-Gear skill called "Chain Rolling" which removes the cooldown of Barrel Roll and allows the I-Gear to continuesly roll incoming missile attacks. Do note that this does not make the I-Gear invulnerable to missile attacks because of the delay between rolls.
There is a common tactic that is useful is some situations where your Gear does not roll sideways, but instead stay in the place and just spin around. This can be done by either rolling left ("A" key) or right ("D" key) then quickly pressing and holding the other key while the Gear rolls. This is very useful in cave maps where there is little space to move around and when rolling may cause you to collide with the terrian/walls.