A Barrel Roll consists of a full roll along a helical path, as if your plane were tracing the inside of a cylinder. In Skyrates it will slightly improve the coverage of your firing arcs while greatly reducing the amount of enemy fire taken.
It should be noted that it is not possible to change aircraft speed when performing this maneuver. The speed you are travelling at determines the side to side motion of the roll - performing one at your plane's maximum speed speed produces an almost perfectly straight flight path, whereas one performed at your plane's stall speed will be wider. Finding which speed suits you the most and is most optimal for coverage is up to you.
Your plane will roll in the direction of the turn most recently taken when triggered with the hotkey. You can roll in either direction by inputting the appropriate key combo, however.