A DAU, or daily active user, is the number of active app users who login or visit your mobile app in one day (24 hour period). This metric is based on the number of users and not individual sessions.
For instance, if a single app user logins 15 app sessions in a 24-day period, the DAU for that specific user will still be calculated as one. This metric is quite important for an organization as it gives a clear picture of the number of users who are engaging with an app during a 24-hour period.
Moreover, you can read more relatively about MAU.