A Loop Block is started at a loop-begin activity and ended at a loop-end activity (the 'brackets'). It indicates that the section of the workflow delimited by the loop-begin and loop-end activities should be repeated as long as the loop condition is met.
The loop-begin activity marks the point to which execution should return if the loop condition is met (i.e. the place to return to if the engine determines that the loop should iterate). The loop-end activity marks the point to which execution should jump if the loop condition is not met.