A Business Rule can execute actions when certain conditions are met. For each business rule you define general information (name, id, tracing, status), one or more trigger points (update or creation of content items, replacing of a file or via schedules), conditions (what should the business rule check e.g. metadata changed), transformations (e.g. to transform unstructured metadata into structured tags) and actions (that will be executed e.g. remove permissions).
Business rules always run in the context of the service user (so FULL privileges to ensure that actions can be carried out), an indication that a business rule might have changed content can be found in the activity panel where the updating user would be the service user.