Microsoft's tool to connect to data, model it, and create interactive reports and dashboards. Power BI Desktop is used to author; the Power BI service (part of Microsoft Fabric) is used to publish, share, create dashboards, and schedule refreshes.
In DP-900 you are asked to choose visuals: lines for trends, bars to compare categories, pie for a few parts of a whole, scatter for correlation, card for a single value, and matrix for crosstabs. A dashboard is a single page of pinned tiles; a report can have several pages.
Power BI is a concept that can show up on the exam. Check the full guide to see it in context.