A bulk action allows for the selection of multiple values from a list, and apply some logic to items that have only been selected. Bulk actions exist for many different Resource Lists, which is what is used for determining items that have been selected.
Bulk actions are usually dispatched as a job, given the bulk nature of the action, which carries out tasks in a sequential manner. There are exceptions to this rule however and Aero can also return responses straight to the browser, e.g. downloading all selected content blocks as a .zip.