As you haven't provided complete information around what metrics you have moved to this chart, I tried the ribbon chart at my end with a simple metric overtime:
In the above chart I have shown Sales overtime split by different clothing categories.
When I hover over the ribbon portion of August 2014, I see a tooltip which ives me following information:
Sales for Shirts for July 2014
Sales for Shirts August 2014
Difference in sales for above 2 months (negative as it is gone down in August as compared to July)
In the month of July, a ranking is created for sales against all the different clothing categories; out of which Shirts generated the most sales and is ranked 1 as compared to other categories on the chart.
Similarly a ranking for the Shirts category for the month of August.
Last but not tht least, a chnage in the ranks for the Shirts category on sales.
I suspect in your case you are moving a proportion metric or a distinct count metric as a percentage. (not clear to me with your screenshot).
You can relate the description from my chart to your's and hopefully the tooltip information is clear to you.