The plugin will look for any relationships between 2 constituents that are unreciprocated. When you are adding a relationship to a constituent record and linking to another constituent,  upon saving and closing, you will be prompted “(Constituent Name) is a constituent. Do you want to reciprocate (Constituent Name) as a relationship for (Constituent Name)?” If you selected No to this prompt, that relationship would not show on the other constituent’s record. Running this plugin will reciprocate the relationship so that it would now show on that constituent’s record.

After running the plugin, a log file is created and opens, displaying the results of the plugin and the constituents by constituent ID's that were reciprocated. 

More information the utility and how to run it can be found in Instructions for downloading and running the Reciprocate All Relationships plug-in.