Daily active user (DAU) measures the total number of users opening a mobile app at least once in a single day. It is about users, not the number of sessions of a single user. Even if one user has more than one session in a day, that user is counted as one in DAU metric.

It is an important mobile app metric to understand the quality of users coming from a single mobile advertising campaign. Comparing the number of installs from an ad campaign and DAU helps mobile advertisers to find the most effective user acquisition channels for their apps. 

DAU also helps marketers to understand the loyalty and engagement of users. Analyzing DAU regularly can let mobile advertisers make data-driven decisions for retention and engagement rates.