If you open up a spreadsheet that you did not create, you may run into the "__xlfn" prefix before certain functions. The generic meaning of xlfn is "unsupported function". Typically, it indicates that the worksheet was created in a newer version of Excel and uses functions not yet available the version currently running.
In the example shown, the UNIQUE function is used to extract unique values from a range of data. UNIQUE is only available in Excel 365, and was introduced in 2020. When the workbook is opened in an older version of Excel, the __xlfn prefix appears. Note that the original result is still displayed. However, the formula will not update to show a new result if data changes.
Solution or workaround
If you open the same workbook in a version of Excel that contains the missing function(s), the _xlfn prefix will be disappear and the function will calculate normally. If this is not possible, you might be able to use a another formula that does not not rely on the newer function.