The Retention feature in App Analysis pulls in retention rates for the selected apps. These retention rates show how long users continue to use the app on a regular basis. Retention gives you the exact percentages of the average user's chance to continue using the app after a specific day.
The graph will show the retention rates from Day 0 all the way to Day 90 for up to 5 selected apps. It is defined as the percent of users who have a session with an application X days after initial installation, it is the exact metric for the the particular day in question.
Below the graph of the Retention curve, there is a table with more details into the data from the graph.
All of this data is also Exportable to a CSV via the blue Download CSV button.