When viewing reports for Today, or This week, or This month, or This year, the reports you are looking at were processed some time ago: for example the reports for « Today » were maybe processed a minute earlier or they could be several hours old. How often reports are processed is configurable in Matomo. It is often important to know how fresh (or stale) the data you’re looking at can be.
To see how long ago was a given report generated, open (for example) the report under Visitors > Devices, and hover your mouse on the first report title. A help icon with a question mark will be displayed, and when you click on this icon it will show the help box. At the bottom of this help box you can see when the report was processed:
Note: the text
Report generated 28 min 31s ago is only displayed when the date range you’re looking at include « Today ».