Cut and brake are safewords for slowing down or stopping a scene if it's too intense or dangerous.
The brake safeword is used to indicate that the person uttering the safeword wants to slow down or change where the game is heading, for instance if the scene is becoming unsafe or uncomfortable. Upon hearing the brake safeword other players must then try to slow down and/or change the approach or direction of the scene.
The cut safeword means stop the scene immediately, and is used if something goes wrong and/or you want to completely stop the scene for whatever reason. When the scene is cut the players involved must find out why the scene was cut and/or how to restart or continue playing.