Record Grouping means that when you add a Multi-Record record which references a Main Record, the record will appear under both of them, just like the "Child Sibling Groups", which link up all the Children together and show all of them on each others forms. (Meaning it doesn't matter whose form the records are entered from.)
This is only for advanced users to setup/enable.
This is enabled by setting the "Advanced | Record Grouping Field" property on the Multi-Record Table. The Field must reference the same Main Entity Type as the Parent-Table, such as the "Organization" or "Child". Only for Multi-Record tables.
For example, to setup a "Branches" Multi-Record table under Organization that links all the Organizations together. Create the Multi-Record table and add a field "Branch" which is a Big-Search List of "Organization", then go to the Advanced Table Properties and set the "Advanced | Record Grouping Field" to use the "Branch" field.