The Break block allows you to break a loop created by the Repeat block. It is to be placed in the "dead branch" of the latter, which represents the path taken by the player when the loop is over.
The Break block almost have a "musical" function. Without it, a Repeat block would create the following pattern : loop*number of iterations, then "dead branch", then loop*number of iterations, then "dead branch"... and so on. The break block breaks this infinite repetition.