The Barrel Roll is a not competition maneuver, as far as aerobatics is concerned. It was used mainly as an evasive/defensive maneuver in combat, particularly WWII. The goal is to force the attacker to overshoot the target. The idea is to shorten the distance a pilot covers in a straight line by adding lateral movement (the distance between the pilot and the attacker). Not so much to slow the plane down, which also works, but sacrificing too much airspeed or altitude for that matter in a dogfight can mean disaster. Also the complexity of the maneuver serves to defeat the guns resolution, at least for a short time until the attacker can correct.
The barrel roll is a combination between a loop and a roll. You basically complete one loop and one roll at the same time. Externally it looks much like a corkscrew lying on a table. While still not a maneuver used in aerobatic competition, it still requires a fair amount of skill to perform well.
In a barrel roll the accelerometer will always (or should) indicate positive G forces, usually between +0.5 and 3.0 G’s.