A dataset is a collection of data used to create a question (such as a calculated question), where it is inserted in place of a variable within the question.
You can create a "private" dataset for a specific question, or a "reusable" dataset that can be used within all questions within a category.
When creating a question that uses datasets, you'll be prompted with two screens.
The first screen allows you to assign a particular dataset to be substituted for each variable.
The second screen allows you to add and remove sets of numbers in the dataset. The numbers used for individual questions presented to students will be selected from this set.