Active users represent the total number of unique game interactions within a set time period, such as daily (DAU), weekly (WAU), or monthly (MAU). In order to calculate this, one must first know what constitutes a viewer/user. The most common way to calculate this would be to look at the number of unique visitors that have paid for/downloaded a game and proceed to make an action within the game.
Metrics such as monthly active users (MAU) or daily active users (DAU) are commonly used to assess a game’s relative success post-launch. Retention is measured by changes between these stats, and is regarded as a better metric to track player stickiness than simply looking at sales/downloads. This is because sales/downloads can, more often than not, be a representation of hype and good marketing and not the game’s ability to hook players, which can ultimately increase their lifetime value.