In Power BI, Aggregations start as tables just like any other table in a model. They can be based off a view or table in the source database, or created in Power BI with Power Query. They can be in Import or Direct Query storage mode.
Once in the model, these tables can be configured so that the engine can use them instead of a detail table to answer queries when possible. The process of creating and configuring aggregations in Power BI is significantly easier than the process of creating aggregations in SSAS multidimensional.
Once an aggregation table is configured, it becomes hidden from end users. Report developers and end users don’t know that it exists and don’t need to change anything in how they query the dataset.
This was one of the key benefits to a multidimensional model. Shabnam has an excellent, detailed article here, so give it a read if you are a Power BI developer.