Daily active users (DAU) is one of the ways used for measuring success of an internet product, e.g., online social games. DAU measures the "stickiness" of an online product by answering the question, "How many unique product users visit the product daily?"
Usually the only requirement for a product user to be considered "active" is that they somehow view the product or engage with the product. Examples of this type of product usage would range from visiting the splash page of the product or commenting on a product post by the product's Facebook page, to actually playing the product itself.
The monthly aggregate of active product users is monthly active users (MAU) of the product.