Run Reaction Action
The Run Reaction action allows you to run another reaction (rule-based or global); this is analogous to a "subroutine" in many programming languages.
The Wait for completion checkbox, when checked, causes the current reaction to pause until the selected reaction has completed execution. By default, this checkbox is unchecked, so execution of the selected reaction begins immediately and may run concurrently with (at the same time as) the reaction that starts it.
If you run a rule-based reaction, and that reaction contains groups with constraints, the contraints are not re-evaluated in that instant, but rather the previous state of the group's constraints as of the last time the rule was evaluated as a whole is used. For this reason, running rule-based reactions that contain constraint groups is not recommended. In fact, although Reactor lets you do it, having a global reaction run a rule-based reaction, or a rule-based reaction run another rule's reaction, is considered dangerous and bad form by the author. Ultimately it will make your logic and reactions more difficult to follow and maintain, so I advise only using this action to run global reactions. I've given you rope, but don't hang yourself with it.
See also: Stop Reaction Action