A display folder is a virtual container used to organize fields (columns), calculated columns, and measures into a single, organized location
Refer: Create a measure table in Power BI
Following these steps to create Display Folder:-
Step-1: Go to the Model tab in Power BI Desktop.

Model Tab Power BI
Step-2: Select a measure or columns that you want to move under a display folder (use ‘Ctrl + click’ for multiple selections), and then press Enter.

Display folder
Step-3: Now you can see all the selected measures and columns under the folder.

Folder
How to create a Nested Display Folders( Sub folder)?
You can also create subfolders to place a backslash (\) in the display folder name. Here is an example:
Step-1: Select the measure that you want to move under a subfolder.
Step-2: Enter the Display folder name (Sales Measures\Subfolder name), exp: Sales Measures\Profit.
Step-3: Press Enter.

Sub folder in Power BI
Step-4: Now you can see the subfolder under Data section.

Display Subfolder
Refer Power BI other post: Power BI tutorials
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I can not delete it afterwards…. how ?
To delete it a folder, I think all that you have to do is to delete all of the references to it. In other words, for each measure where you created a subfolder, go into Properties and clear the contents of the Display Folder box.
But how can I delete a folder?
To delete it a folder, I think all that you have to do is to delete all of the references to it. In other words, for each measure where you created a subfolder, go into Properties and clear the contents of the Display Folder box.
I cannot find any clear way to remove an unwanted folder in the newest version of PBI, however I found that by selecting the measures contained within the existing folder, then deleting/clearing the ‘Display folder’ text box in the Properties pane, it will remove the folder from the Field pane. Seems odd there is no discrete way to remove the folder from the data model (watching tutorials it seems it was a feature at one point? A simple right click on the folder and ‘remove from data model’ was an option)
Say you have 3 fields under a folder f1. Select all through fields holding ctrl or shift,
Delete the Display folder name using backspace/del key.
Voila . All deleted at once . No individual deletes
I would find this idea a lot more appealing if you could directly add new measures and columns to these subfolders instead of having to first create them at the top level and then move them.