An impression is a single instance of a document page put onto a piece of paper. For example, if you print 4 pages of a document on a single sheet of paper, that's 4 impressions but only 1 sheet.
A sheet is a single physical piece of paper. It's possible to print multiple document pages using a single sheet (e.g., by printing double-sided, or by configuring multiple impressions per page).
You're introduced to some of the common terms that will be used in working with SAP workloads on Azure. Prepare for Exam AZ-120 Planning and Administering Microsoft Azure for SAP Workloads.