The Completion Rate formula on a Story is based on the number of people who have completed your Story.
It allows you to understand which percentage of users have watched your Story until the end.
Here is an example of how this is calculated:
Impressions of Story = 1323
Exits = 111
1323 - 111 = 1212 (This tells you the number of completions)
(1212 / 1323) x 100 = 92%
Completion rate = 92%