A batch audience is used to retrieve a set of records from the data warehouse. Having done so, typically you will then undertake some form of fulfillment activity in respect of the data set thus created (for example, sending an email).
The rules used to determine which records are to form the dataset are defined by the audience file with which the batch audience is configured. For full details of audiences, please see the Audience Designer documentation.
Note that a batch audience contrasts with an interactive activity configured with an audience insofar as a batch audience runs the rules defined by the audience once only. The interactive activity may run the rules defined by its audience a number of times, in accordance with its frequency settings, to generate its dataset cumulatively.