The retention wheel tells you the number of donors who have given at least once during the two 12-month periods that make up the most recent 24 months.
The larger number is donors that gave between 12 and 24 months ago.
The smaller number is donors (out of the larger number above) that gave again within the most recent 12 months.
The retention wheel operates on a rolling 12 and 24 month cycle, thus it compares the two most recent 12-month cycles and changes every day. If a new donor makes a donation today, that donor is not factored into the retention wheel until one full year from today. Essentially, a year must pass before a donor can be retained.
For more information, view Understanding the Donor Retention Wheel.