As the names suggest:

• Most recent only (MRO) : only the most recent edits to a page. E.g. if there is one edit by XX and then an edit by YY, only the YY edit is listed, as it is the most recent.
• flat: just like RecentChanges, but as RSS doesn't support hierarchies, the thread view is flattened out, i.e. they appear as equal siblings next to each other (taking the example from above, the RSS shows edit of YY, then edit of XX {reverse chronological order})