A Power BI item, formerly called a dataset, that contains the tables, relationships, DAX measures, hierarchies, and row-level security roles that reports use.
In DP-900: DAX measures are calculated at query time based on the report's filters, while calculated columns are computed and stored when the model refreshes. Hierarchies (Year > Quarter > Month) let users drill into data.
Power BI semantic model is a concept that can show up on the exam. Check the full guide to see it in context.