A Guard Break is a move which forces enemies to stop guarding and stuns them for a short time. When a Guard Break comes in contact with a guarding opponent, a shattering sound is heard, and the opponent is unable to perform any action except a properly-timed Guard Impact. These attacks often are significantly more damaging to the opponent's Soul Gauge than regular attacks. Guard Breaks are characterized by the blue streaks of lightning which coat the user's weapon while the move is performed (equivalent to the flame effect iconic of [Unblockable] attacks), and they are indicated on a character's move list with the blue "GB" tag.
Guard Breaks, in exchange for their powerful effect, are often slower to execute, and leave the user vulnerable before and after the swing. Ironically, the slow execution allows sufficient time for an opponent to resume guarding. However, Guard Breaks are still valued for their high damage output to both the opponent's health and Soul Gauge. Some Guard Breaks have quick recovery time, which usually means that the attacker can easily land a hit before the opponent can resume guarding.
Oddly, some combos and attack throws include Guard Breaks, though the opponent is not allowed to guard during these times. The moves are probably included for the aesthetic benefit of the lightning effect.