The percentage of active users that keep using your app month after month. For instance if you have 10,000 active users and a retention rate of 80%, at the end of the month, 10,000 x 80% = 8,000 users will remain active in the following month.
This is a metric very similar to the churn rate and very important to take into consideration. This is usually a big mistake that many do when creating projections or plans for their app. They don't take into consideration that some users will stop using the app after a while.
As for this number, it may vary highly. Some free apps have a retention rate of just 15% while other giants like Facebook have a retention rate of more than 90%. My advice is to be very careful and play it on the safe side, taking a low number when creating a financial model or using to make projections.